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F R I D A Y , M A Y 2 4 , 2 0 0 2 TODAY’S EVENTS Eric Lander on genome Eric Lander, founder and director of the Whitehead Center for Genome Research, will give a talk on “The Human Genome and Beyond” at the Friday lecture today (May 24) at 3:45 p.m. in Caspary Auditorium. A member of the Whitehead Institute and professor of biology at The Massachusetts Institute of Technology, Lander has been one of the principal leaders of the Human Genome Project. Lander is a geneticist, molecular biologist and mathematician, with research interests in human genetics, mouse genetics, population genetics and computational and mathematical methods in biology. Lander and his research group have developed many of the tools of modern genome research — including genomic maps of the human, mouse and rat genomes in connection with the Human Genome Project and techniques for genetic analyses of complex, multigenic traits. He has applied these techniques to understanding cancer, diabetes, hypertension, renal failure and dwarfism. He first came to the Whitehead Institute as a Whitehead Fellow in 1986, while still an assistant professor of managerial economics at the Harvard Business School. Lander was a Rhodes Scholar and received his D.Phil. in mathematics from Oxford University in 1981. Lander’s many honors and awards include the MacArthur Foundation Prize Fellowship and the Woodrow Wilson Prize from Princeton University. A fellow of the American Association for the Advancement of Science, Lander is a member of the National Academy of Sciences and its Institute of Medicine. news notes THE NEWSLETTER FROM THE ROCKEFELLER UNIVERSITY’S OFFICE OF COMMUNICATIONS AND PUBLIC AFFAIRS 3-D images shed light on first steps of RNA synthesis The first three-dimensional images of the initiating form of the molecular machinery in bacteria that “transcribes” genetic information from DNA into RNA — the crucial first step for making proteins — is reported in a pair of papers in the May 17 issue of the journal Science. used a technique called X-ray crystallography to reveal the structure of the holoenzyme, the core enzyme bound to the sigma subunit.They also used this research tool to visualize the structure of the holoenzyme bound to a short piece of DNA containing a promoter sequence. These research findings of The Rockefeller University scientists illuminate how the process of transcription begins in bacteria. Understanding this process may provide clues for developing new drugs to treat bacterial infections. In X-ray crystallography, X-rays are fired at crystallized proteins, or complexes, of proteins bound to other molecules. Analysis of the data from the diffracted Xrays yields information about the three-dimensional structure of the molecule. Some molecules can be visualized at the resolution of a few angstroms (a typical atom is roughly two angstroms in diameter). But Darst notes that the crystals his group used did not diffract well enough to produce a high resolution structure. So, he and his colleagues used a novel hybrid approach. The DNA-transcribing machinery is called the RNA polymerase (RNAP).The RNAP comprises a core enzyme, which can synthesize RNA from a DNA template, but cannot initiate transcription by itself. Initiation requires another protein, the sigma subunit, which binds the core enzyme to form the holoenzyme. The sigma subunit “tells” RNAP where in the genomic DNA to start transcription. It does this by recognizing a section of the gene’s DNA called the promoter region and “melting” the DNA to expose one of the double-helical strands the RNAP will use as the template to synthesize RNA. The Rockefeller University researchers, led by Seth Darst, studied RNAP from a bacterium called Thermus aquaticus, a member of a family of organisms that thrive at high temperatures.They “From previous and parallel work in our lab, we had high-resolution structures of the RNA polymerase and parts of the sigma subunit,” says Darst, who is the Jack Fishman Professor and head of laboratory at Rockefeller.“We fitted the high-resolution structures into the lower-resolution structure to obtain the structure of the holoenzyme. And since the structure of DNA is known, we used that to solve the structure of the holoenzyme bound to the promoter fragment.” continued on page 2 These are the first 3-D images of the molecular machinery in bacteria that initiates the process of transcribing DNA into RNA. A short piece of DNA containing a promoter sequence (green and white double helix) binds to a bacterial “holoenzyme” (multicolored structures). In the top photo, the holoenzyme specifically recognizes the sequence of the promoter and forms a structure called a closed complex — closed because the DNA is not “melted.” Once the sequence is recognized, the holoenzyme goes through a series of steps and actually melts DNA (bottom photo) to expose one of the double-helical strands the core enzyme will use as the template to synthesize RNA. These structures “provide fuel for a model to understand the steps in bacterial transcription.” Leprosy bug provides clues to early nerve degeneration Possible insight to multiple sclerosis and neurodegenerative diseases that destroy nerve cell “insulation” Scientists at The Rockefeller University and New York University School of Medicine have found that the nerve damage that leads to a loss of sensation and disability in people with leprosy occurs in the early stages of infection. The finding was reported in the May 3 issue of Science. The nerve damage, a hallmark of leprosy previously thought to be a byproduct of the immune system’s response to the leprosy bacteria, now is thought to be a direct result of the leprosy bug attaching itself to specialized nerve cells — Schwann cells — the glial, or supporting, cells of the peripheral nervous system (PNS). The findings suggest that the body’s immune response does not play a significant role in the early stage of neurological injury.The damage is characterized by the disruption of the myelin sheath, the insulation on nerve cell connections that helps transmit rapid signals between the brain and the peripheral organs, for example, skin and muscles. Myelin is arranged in segments around axons and sep- arated by nodes. At and near the nodes, sodium and potassium channels are clustered in high density in the axon membrane so that they can transmit an electric charge called the action potential. Damage to myelin dramatically decreases such action potential and nerve conduction velocity, and eventually causes loss of sensation, disability and paralysis. Using laboratory cell tissue cultures and mice genetically manipulated to lack two key immune system cells, the research team, led by Rockefeller University micro- biologist/cell biologist Anura Rambukkana, showed that Mycobacterium leprae (M. leprae), the bacterium that causes leprosy, destroys the protective myelin sheath that surrounds nerve fibers and then hides out in the supporting cells that enclose nonmyelin nerve fibers, poised to initiate later attacks. “What we show here is a novel mechanism of inducing demyelination by a bacterial pathogen,” says Rambukkana, principal author and research assistant professor in the Laboratory of continued on page 2 RNA synthesis continued goes inside the polymerase toward the active site and back out, producing a very unusual protein-protein interaction. Seth Darst and his team used X-ray crystallography to visualize the machinery in RNA synthesis. The holoenzyme structure offers some clues to how transcription begins in bacteria. Research from Darst’s lab published recently in Molecular Cell revealed that the sigma subunit contains four protein “domains” connected to one another by short stretches of amino acids called “linkers.”With information from the holoenzyme structure published in Science, the researchers show that the domains of the sigma subunit spread out and bind to one face of the core enzyme, which positions sigma to bind to DNA. The most interesting finding, says Darst, involves the position of one of the linkers.This linker, containing about 33 amino acids, is much longer than the others.This linker “These sigma domains sit on the surface of the RNA polymerase, but this very long linker is buried inside the polymerase, near the active site of the enzyme,” he says.“In order to establish this complex, there need to be a lot of conformational changes to get the linker inside.” In bacteria, the first step in transcription is binding of RNA polymerase to a nucleotide, one of the four building blocks that make up DNA.The researchers hypothesize that the linker participates in binding the initiating nucleotide, and once the RNA polymerase begins synthesizing RNA, the sigma subunit is no longer needed. the path of the RNA,” says Darst. “As the RNA transcript begins to be elongated, it has to push this linker out from a channel inside the polymerase.The linker’s not completely pushed out until the RNA becomes about 12 nucleotides long.” Scientists have known from biochemical experiments that a transition occurs when the RNA transcript reaches a length of 12 nucleotides: the complex becomes much more stable and the elongation phase begins. “We think that signal to go into elongation occurs when the RNA becomes long enough to fill the channel,” says Darst.“It pushes the linker peptide out, and then sigma begins to fall off.” Gene expression is regulated at every step, but probably the major focal point of regulation is at the initiation of transcription. According to Darst, the linker that is buried near the active site may play a role in telling the RNA polymerase when to start synthesizing RNA. This structure also explains a curious phenomenon called abortive initiation, in which transcription starts and stops. The polymerase remains at the promoter site synthesizing short pieces of RNA, which fall out, and then the process starts over again. This can happen hundreds of times before the process moves on and a long RNA transcript is synthesized. “The linker sticks inside the polymerase and actually blocks From the structural information and other experiments, the says Salzer.“Such signals could also be activated in other diseases that cause demyelination.” nerve injury in patients before the immune system comes into play. The researchers also showed that M. leprae does not need to be alive to demyelinate nerve cells. Similar results were obtained after cultured Schwann cells were exposed to bacteria that had been killed with radiation and to fractions of the bacterium’s cell wall. Schwann cells also play a significant secondary role in providing support for the various growth factors during the development of the spinal motor neurons and sensory neurons. Thus, Schwann cells play a wider role not only in the normal development of the nerve cells in the PNS, but also in their regeneration. researchers now know that abortive initiation happens because of a competition between RNA and the linker peptide. If the peptide wins, the RNA falls out and transcription starts over. “Every once in a while the RNA manages to push the peptide all the way out, and once it’s long enough to push it all the way out, it’s done, and there’s no more abortive initiation,” says Darst. The structure of the holoenzyme bound to the promoter fragment, Darst says, “provides fuel for a model to understand the steps in bacterial transcription.” The holoenzyme specifically recognizes the sequence of the promoter and forms a structure called a closed complex — closed because the DNA is not melted. Once the sequence is recognized, the holoenzyme goes through a series of steps and ultimately forms the open complex, where it actually melts DNA. “From this structure and others, we made a model of the closed complex and the open complex, kind of a beginning and an end,” says Darst. “What we’re really interested in now is the in-between steps. “We think the models are pretty good, but they don’t provide a lot of insight into how it gets from A to B.” A better understanding of transcription in bacteria could lead to new drug targets to treat bacterial infections, particularly those that are resistant to current antibiotics. Previous research from Darst’s lab showed that rifampicin, one of the two drugs most effective against tuberculosis, kills the microbe that causes TB by physically blocking that bacterium’s RNA polymerase. The Rockefeller scientists who conducted this research are postdoctoral fellow Katsuhiko S.Murakami, research scientist Shoko Masuda, postdoctoral fellow Elizabeth A. Campbell and research scientist Oriana Muzzin. This research was supported in part by the National Institute of General Medical Sciences, part of the federal government’s National Institutes of Health, the Norman and Rosita Winston Foundation and the Human Frontiers Sciences Program. — Joseph Bonner Leprosy continued Bacterial Pathogenesis.“This may unravel clues for early molecular events of neurodegeneration processes in other diseases, such as multiple sclerosis, which we currently know nothing about.” “By using this bacterium we will be able to obtain novel insight not only into the mechanism of the early demyelination process, but also how the complex molecular architecture of the myelinated fiber is disrupted,” adds Rambukkana. Using a “co-culture” system developed by co-author James Salzer at the NYU School of Medicine, in which myelinated nerves form normally in cell culture, the researchers found that M. leprae produced significant damage to the myelin sheaths 24 hours after attaching to the nerves. Myelin damage is the earliest effect observed, followed by the degeneration of nerve axons that carry the nerve impulses. M. leprae does not harm any other parts of the cell, and there is no sign that it causes cell death. The researchers found that, unexpectedly, M. leprae does not need to enter the cell to cause degeneration of the myelin sheath. “This suggests that binding of M. leprae to the surface of the myelin sheath is sufficient to induce myelin breakdown, presumably by activating signals inside the cell,” 2 Previous research by Rambukkana, Salzer and their colleagues implicated a major component of the bacterium’s cell wall called PGL-1 in its predilection for peripheral nerves, and they now propose that PGL-1 is a crucial cell wall molecule directly involved in nerve damage in leprosy. The mouse model the researchers studied is called a “Rag-1 knockout.”This mouse, developed by Associate Professor Peter Mombaerts in the early 1990s as a postdoc in Nobel laureate Susumu Tonegawa’s laboratory at MIT, is genetically altered to lack mature B and T cells (B cells are responsible for producing the infection-fighting proteins called antibodies, while T cells help regulate the body’s immune response). As in the cell culture model, direct administration of both M. leprae and its cell wall to the peripheral nerves of Rag-1 mice caused significant myelin damage, providing clues for early Nerve cells in the PNS Using myelin-forming primary nerve tissue cultures (top left), and mice genetically manipulated to lack two key immune system cells (bottom left, electron micrograph), researchers led by are able to regenerate Rockefeller microbiologist/cell biologist Anura Rambukkana showed that Mycobacterium leprae, after injury, unlike their the bacterium that causes leprosy, selectively destroys the protective myelin sheath that insulates counterparts in the cen- nerve fibers (right-hand panels). tral nervous system (CNS). Examination of M. leprae is known to cause debil- The knowledge gained by such myelinated and non-myelinated M. leprae-induced myelin damage nerve cells infected with M. leprae itating neurological injury in humans, but the clinical manifesin the early infectious process revealed an interesting survival tation occurs years after a slow provides valuable insights into the strategy of this bacterium. As the infectious process. According to pathologic mechanisms of early damaged myelinated nerve cells Rambukkana, information about neurodegenerative diseases in repair themselves by generating the pre-clinical mechanisms in general. new nonmyelinating Schwann M. leprae–induced demyelination cells after attack, M. leprae Rambukkana’s and Salzer’s comay allow researchers to develop sequesters itself in these nonauthors are George Zanazzi at therapeutics and common diagmyelinated nerve cells, waiting NYU and Nikos Tapinos at for a chance to attack again, once nostic tests for early detection of Rockefeller. demylenating diseases of both they multiply and escape from — Joseph Bonner infectious origin and unknown these cells. Rambukkana hypothetiology, such as multiple sclerosis esizes that this phenomenon accounts for the lapsing/remitting and Guillain-Barré syndrome. characteristic of leprosy. news notes M AY 2 4 , 2 0 0 2 NEWS BRIEFS Acting President Sakmar comments on research facilities Symposium recognizes Bob Roeder, transcription pioneer Editor’s note: Acting President Thomas P. Sakmar responded to several questions from News&Notes about the recent decisions of the Board of Trustees’ Executive Committee about the proposed 12-story research tower on the north campus and the large-scale Roosevelt Island housing project for faculty and students. On May 17, Board Chairman Richard Fisher sent to the Board a brief report about the actions taken at the committee’s meeting May 8. number of labs from 72 today to 100.The added people and resources needed to support such an expansion would have dramatically changed the university and significantly increased the university’s overall operating budget. In the report, he wrote that the committee had “resolved not to proceed further with this particular plan for a new research facility, while recognizing that we may develop alternative plans for a better supported and more affordable facility on this prime building site in the future.We also resolved to end our consideration of the large-scale Roosevelt Island Housing Project.” Since Feb. 11, when I became Acting President, over $5 million has been contributed by individuals to signal their support for the university and our research and educational programs. As we near the end of the fourth year of our six-year Centennial Campaign, the university has raised $278 million toward its $350 million goal. This is impressive; it is possible that we may complete the campaign ahead of schedule. To honor Robert Roeder on his 60th birthday, about 25 colleagues, including former graduate students and postdocs from his lab, have organized a symposium, “Three Decades Studying the Regulation of Eukaryotic Transcription,” which will occur 9 a.m. to 5:30 p.m., Friday, May 31, in Caspary Auditorium. As head of the Laboratory of Biochemistry and Molecular Biology, Roeder has significantly improved scientific understanding of how human genes are switched “on” or “off ” — a crucial life process occurring at all times in every one of the billions of cells comprising the human body. Recent research finding in his lab are described in an article on page 5. Lectures at the symposium will reflect on his initial groundbreaking discovery of the three proteins responsible for “reading” human genes as well as his ongoing elucidation of the complex manner in which this vital process is controlled. The speakers will focus on the four major areas in transcription that are the targets of Roeder’s lab: activators, general transcription factors, chromatin and signaling. A complete schedule can be found at www.rockefeller.edu/lectures/ 053102.html. “Designing for the Senses” No, it’s not an extra who wandered off the set of Star Wars. It’s Paul Smith, staff member in Communications and Public Affairs, trying on a “cocoon suit” designed by Eiko Ishioka for Olympic athletes. The suit, which was used during the 2002 Winter Olympics in Salt Lake City, shuts out sound and other stimulation and helps athletes concentrate on their performances. The suit was exhibited at “Designing for the Senses,” a day-long conference that brought together experts in science and technology and the design, culinary and fashion professions.The May 17 event was co-sponsored by The Rockefeller University and The Cooper-Hewitt, National Design Museum of the Smithsonian Institution. PEOPLE IN THE NEWS Is it true that the new research tower project is being set aside? As it is now proposed, yes. However, a new plan for a research building and for modernization of current research laboratories may emerge from the evaluation of all of the existing research facilities and research infrastructure. The Executive Committee of the Board has directed me to begin leading this evaluation process. Why was the proposed research building reconsidered? It was reconsidered in part because of the recent updates of estimated construction budgets for the proposed building. These costs are substantially higher than the estimates when the Board approved the concept for the facility. The proposed research building, if erected, would have substantially, and not incrementally, transformed the university. It was based on a program that would have increased the university’s lab space significantly and brought the total Is there a problem with fund raising — is that the reason for the decision? No. In fact fund raising has been very successful thus far this year. How will the Executive Committee’s decision affect the search for the next president and faculty recruitment in general? It’s not likely to have any effect at all. The university has adequate space for its current faculty and for the active searches under way. For example, about 24,000 square feet of new space is coming on line in the Rockefeller Research Building and another 7,000 square feet in the hospital. Looking forward, we are in very good shape with respect to funds available for the new faculty recruitment and faculty development. How did Arnold Levine’s resignation affect this decision? When I took over as Acting President, I fully intended to implement the plans that I had inherited. However, the Board instructed me to carry out a careful analysis of the capital project including the Roosevelt Island housing development project. Considering that a change in the administration had occurred, it was natural to take a fresh look at our current and future needs and see how much they would cost. This is a normal process for organizations under new leadership — a fresh perspective is always welcome. When presented with the upto-date picture, the Executive Committee of the Board directed me to act accordingly. How will this decision affect the university’s research future? This decision was made to insure that the university would continue to have the resources and unique environment to provide its excellent faculty and students with the resources needed to make the stellar scientific achievements for which The Rockefeller University is well known. What role did the faculty play in this decision? The faculty did not play a direct role in the Executive Committee’s decision. But, they will play a very strong role in the process of determining our future direction and the research resources that this campus needs for the future. It is after all the scientists who are responsible for carrying out our scientific research mission. The administrative staff and scientific support staff also will be called on to analyze the effects of any major changes in our research program. What about housing? Although the proposed major Roosevelt Island housing development project is being set aside, the Executive Committee directed me to act promptly to insure that we will have excellent housing options for our students, postdocs and faculty. We are now actively addressing this issue. — Cathy Yarbrough Cheers on their 10th and 20th anniversaries Postdoctoral fellowships The university’s Faculty Nominating Committee has selected the recipients of the following postdoctoral fellowships for the coming academic year. Charles H. Revson Fellowships in Biomedical Research: Deepti Jain (Seth Darst lab), Terry Lechler (Elaine Fuchs lab), Vincent Noireaux (Albert Libchaber lab) and Lei Zheng (Robert Roeder lab). FPO Women & Science Postdoctoral Fellowships: Sara Buonomo (Titia de Lange lab), Emily Harms (Michael Young lab), Li Liu (Magda Konarska lab) and Woelsung Yi (Jeffrey Ravetch lab). The 2002-2003 C. H. Li Memorial Scholarship recipient is Minghao Zhong (James Darnell lab).The committee also selected Michael Lampson (Tarun Kapoor lab) as the first recipient of the Francis Goelet Fellowship, a new three-year award. 3 Massive applause and cheers erupted as Acting President Thomas P. Sakmar announced the names of each faculty and staff member who had served 10 and 20 years at the university. An article about the May 16 reception for Employee Recognition Day, and the employees honored, will be in the next issue of News&Notes. More than just packaging, histones help turn on genes Rockefeller scientists redefine role of proteins responsible for bundling DNA into cells Histones, the proteins that help roll several feet of DNA into the microscopic span of a single nucleus, are turning out to be much more than just packaging material. Instead, recent studies indicate that these once underrated proteins actively participate in switching genes “on” — a vital life process occurring at all times in each one of our cells. Now, new research at The Rockefeller University shows that the dangling ends or “tails” of histones also are less passive than previously believed. In fact, the researchers say these tails act more like “arms” to directly affect gene activity once the DNA is unpackaged. The findings, reported in the April issue of Molecular Cell, not only provide fundamental insight into the complex workings of the human cell, but also may one day lead to new treatments for diseases in which genes have been improperly turned on or off, such as cancer. Furthermore, this kind of knowledge may provide new strategies for combating the handful of developmental disor- patterned, with varying combinations of chemical tags that ultimately dictate which genes get switched on, and which remain silent. “If there is some form of histone code, then this new system could potentially enable scientists to interpret it,” says Robert G. Roeder, head of the Laboratory of Biochemistry and Molecular Biology at Rockefeller and principal author of the report. Moreover, knowledge of such a code would allow scientists to manipulate it for the treatment of diseases in which gene production has gone awry. Unraveling DNA Histones help to package DNA, the hereditary material of life, into each cell’s nucleus.The double-helical strand of DNA wraps around a ball of histones consisting of four distinct proteins: H2A, H2B, H3 and H4. This fundamental unit, called a nucleosome, is repeated at regular intervals throughout the length of DNA and, under a microscope, resembles “beads on In the laboratory of Robert G. Roeder, Woojin An (left) and Vikas B. Palhan (right) report that the “tails” of histones, previously thought to play only a passive role in packaging DNA, in fact, behave more like “arms” to switch on genes once the DNA becomes unpackaged. ders associated with malfunctioning histones, such as Rubinstein-Taybi syndrome and Coffin-Lowry syndrome. “We found that if we removed the tails from histones, genes could not be activated,” says Woojin An, a postdoctoral fellow at Rockefeller and first author of the new research paper. “Because these tails are essential to switching genes on, they are really more like arms.” In addition to this unexpected finding, the researchers also have developed a powerful new biochemical tool for deciphering a possible “histone code.” According to this emerging theory, histone tails are coded, or 5 a string.” Compact strings of nucleosomes coil up further to form chromatin and even further to become the familiar X-shaped chromosomes of human cells. Hidden within this bundled mass of histones and DNA are the genes: the cell’s instructions for manufacturing its tens of thousands of proteins. Therefore, before the cell’s machinery can “read” or, as scientists say, “transcribe” these genetic instructions and ultimately produce a protein of interest, it must somehow open up or unfold the tightly wound chromatin. But just how the cell’s machinery knows which stretches of chromatin to break open and Rockefeller researchers have developed one of the first successful test tube systems for studying the effects of DNA packaging proteins — histones — on gene activity. Above: reconstituted “chromatin” containing genetically altered histones and DNA. The new chromatin system led the researchers to the finding that “tails” of histones are essential to switching on genes. In addition, the system will allow scientists to explore the possibility of a “histone code” — an emerging theory that states that varying patterns of chemical tags along histone tails control which genes become activated, and which remain silent. read, and which to keep silent, remains poorly understood. Some scientists think that the key to regulating the activity of genes may lie with the very proteins that package them. They propose that the tails of each histone become coded with different patterns of chemical tags and thus, like a combination lock guarding a safe full of genes, only permit access to those gene-reading machines that somehow “know” the right “histone code.” This code is not inherent to the histones themselves but is generated by other proteins in the nucleus. Now, as a twist to this developing story, Roeder and collegues have discovered an entirely novel function for tails.They show that these dangling ends, in addition to repressing genes through chromatin folding, also play an active role in switching genes on once the folding is reversed. “As counterintuitive as it may seem, these tails seem to act both to block and to activate gene transcription,” says Roeder. Previously scientists thought that the tails — which, unlike the rest of the histone, extend from the compact body of the core nucleosome — blocked transcription of a gene by causing chromatin folding. Gene activation was thought to occur after the inhibitory effects of the tails were neutralized by the addition of a chemical tag — a process called acetylation. “Histone tails are covered with positive charges and are therefore attracted to the negatively charged DNA,” says An. “Scientists believed that acetylation, by masking the positive charges on tails, caused the chromatin and underlying DNA to become less constricted by the histones and thus more receptive to unfolding.” “Something else must be repressing transcription of genes besides the tails,” says An. “And this must be the residual nucleosome core structure.” A novel biochemical tool “In addition,” he says, “these data tell us that the tails are required for transcription, a surprising finding really when you consider that for several decades, these protein tails were thought to be mainly involved in repressing genes.” To test this theory, the researchers recreated coiled-up chromatin in a test tube using engineered or recombinant histones and DNA — a feat in itself considering that this type of biochemical tool had never before been successfully implemented.They also showed that by adding a specific gene activator protein and a “coactivator” protein called p300 they could coax the chromatin to open up and permit transcription of a target gene. Both a gene activator and a coactivator are required to switch on a target gene. First the activator binds to a specific sequence of DNA in front of the target gene; next the coactivator binds to the activator and allows it to communicate with the gene-reading or transcription machinery, and finally transcription begins. Importantly, p300, in addition to its role as a mediator, enhances transcription via acetylation of histone tails. Next, the researchers repeated the chromatin assembly experiment with recombinant histones lacking tails. Because the prevailing theory at the time stated that the physical presence of histone tails acts mainly to prevent transcription of a target gene, they predicted that the tailless histones would render chromatin unable to fold and automatically lead to gene activation. But, to their surprise, the exact opposite was true: the chromatin, though unfolded, remained silent and unfit for gene transcription. The researchers then asked how acetylation of histone tails fits into their new model. If this process of chemical tagging does not simply modify the tails to enable chromatin unfolding, they wondered, then what is its role? To address this question, they again assembled chromatin in a test tube; only this time they used histones with tails genetically altered to make acetylation by p300 impossible. Once more, the researchers found that the target gene could not be activated, thereby demonstrating not only that histone tails are required for turning genes on, but, more specifically, that acetylated histone tails are required. Finally, taking this experiment one step further, the researchers showed that p300 prefers to acetylate histone protein H3, with its next choice being H4. In addition, they found that only a few of the possible acetylation sites on histone tails H3 and H4 seem to be essential for gene activation. — Whitney Clavin