Download Regulation of Heat-Shock Response in Bacteria

Regulation of Heat-Shock Response
in Bacteria
GIL SEGALa AND ELIORA Z. RONb
Department of Molecular Microbiology and Biotechnology, Tel-Aviv
University, Tel-Aviv, Israel
The heat-shock response is a widespread phenomenon found in all living cells. It
is characterized by the induction of many proteins in response to change in temperature. The same proteins are also induced by a variety of environmental stress
conditions, such as the addition of ethanol or heavy metals. Therefore, it can be
concluded that this response is a stress response and not only a heat-shock
response. The group of proteins involved in the heat-shock response, called heatshock proteins, includes chaperons, proteases, and regulatory factors. Among
these the best studied genes and proteins are the two major chaperons Hsp60,
encoded by the bacterial groEL gene,1 and Hsp70, encoded by the dnaK gene.2
The first studies on bacterial heat-shock response were performed in
Escherichia coli K-12. In this strain the heat-shock operons have specific heat-shock
promoters, recognized by the heat-shock sigma factor—σ32—that acts as a transcriptional activator.3,4 This sigma factor has a short half-life, being degraded by a
specific protease, the product of the hflB ( ftsH) gene.5,6 Damaged proteins, produced upon a shift to a higher temperature or exposure to other conditions that
bring about protein denaturation, initiate a cascade of events that brings about
stabilization of σ32 and preferential expression of heat-shock genes. (For review,
see references 7, 8, and 9.)
In the last few years it has become clear that the E. coli heat-shock response is
a simplified version of heat-shock response in other bacteria. It now appears that
the phylogenetic groups of γ2 and γ3 purple bacteria, which include bacteria such
as E. coli, Pseudomonas aeruginosa, and Vibrio cholera, are unique in that all their
heat-shock genes appear to be controlled by σ32 and no other specific regulatory
element has been detected.3,4,10–12 In other eubacterial groups there are several control mechanisms that regulate the expression of heat-shock genes. For example, in
the gram-positive Bacillus subtilis there are at least three groups of heat-shock
genes, only one of which is activated by a heat-shock sigma factor—sigma B.13
One of the recent findings relating to the control of the heat-shock response in
eubacteria is the existence of an inverted repeat (TTAGCACTC-N9-GAGTGCTAA)
located at the upstream regulatory region of heat-shock operons (see SCHEME 1).
So far, it has been found only in the upstream region of groE, dnaK, and dnaJ operons or genes, coding for the major chaperons. However, it should be stressed that
since these major chaperons are highly significant in bacterial physiology and
pathogenicity (they constitute dominant antigens) the genes coding for them have
been analyzed extensively, and sequence data are available from bacteria belonging to most phylogenetic groups. Because not much is known about other heatshock genes, it is still impossible to determine if the inverted repeat (IR) is unique
to operons coding for chaperons.
a
Correspondent author: Fax: 972 3 6414138; e-mail: [email protected]
Present address: Department of Microbiology, Columbia University, 701 West 168th
Street, New York, NY 10032.
b
147
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ANNALS NEW YORK ACADEMY OF SCIENCES
The inverted repeat (IR, CIRCE14) was detected in a large number of phylogenetically distant bacteria (for review, see Segal and Ron15) and is highly conserved.
The eubacterial groups in which the conserved IR has been detected include
high and low G+C gram-positive bacteria, Mycoplasma, Chlamydia, Cyanobacteria,
Spirochaetes, and many gram-negative bacteria of the α, β, and γ1 purple proteobacteria.15–19 The only groups where it is probably not present at all are the γ2
and γ3 purple bacteria that—as mentioned before—use only the σ32 transcription
activation. In all cases studied so far, the IR acts as a binding site for a protein
repressor—Orf39 (or OrfA, in B. subtilis) the product of the hcrA gene. Deletion of
the IR results in constitutive expression of the operon.20–24 The bacteria that use the
IR as a control element can be divided into two groups in respect to the types of
regulatory systems. The first regulatory system has been demonstrated in several
gram-positive bacteria of the low G+C group. In these bacteria all the operons
coding for chaperons are transcribed by the vegetative sigma factor—σ70 or sigma
A—and the IR is the only detected regulatory element.13,14,16,23
The second regulatory system has been demonstrated in bacteria belonging to
the α-purple proteobacteria, Agrobacterium tumefaciens, Bradyhizobium japonicum,
and Caulobacter crescentus. In these bacteria the heat-shock operons contain a specific heat-shock promoter that is unique and different from the vegetative promoter and from the heat-shock promoter of E. coli.25 This promoter is recognized
by a heat-shock sigma factor—σ32-like factor—that activates the genes.26–30
Several—but not all—of the heat-shock operons contain, in addition, the conserved IR and respond to the Orf39 repressor (FIG. 1).20,25,31,32
NNN
N
N
N
N
N N
CG
TA
CG
AT
CG
GC
AT
TA
TA
SCHEME 1. The conserved inverted repeat (IR, CIRCE) of heat-shock operons.
SEGAL & RON: REGULATION OF HEAT-SHOCK RESPONSE
149
The two control mechanisms described above act at the level of transcription.
In addition, there is evidence for two types of post-transcriptional control. The IR,
when transcribed, is involved in control of transcript stability. In the groE operon
of B. subtilis and A. tumefacience deletion of the IR results in a longer half-life of the
transcript under non-heat-shock conditions.22,23
An additional post-transcriptional control mechanism was demonstrated in A.
tumefaciens.33 and involves specific cleavage of the groESL operon transcript. The
resulting groES transcript is rapidly degraded, while the groEL transcript is stable,
leading to differential expression of the two genes of the operon. This mRNA processing is temperature-dependent and constitutes the first example of a controlled
processing of transcripts in bacteria.
SUMMARY
Stress response in bacteria is essential for effective adaptation to changes in the
environment, as well as to the changes in the physiological state of the bacterial
culture itself. This response is mediated by global regulatory mechanisms affecting several pathways. It now appears that these regulatory mechanisms operate
by transcriptional control, translational control, and proteolysis. One example to
be discussed extensively is the heat-shock response. In Escherichia coli, where it has
been studied initially and most extensively, the expression of the heat-shock
operon is transcriptionally controlled by the employment of the heat-shock transcription factor σ32, that recognizes specific heat-shock promoters. Later studies
indicated that in most bacteria the control of the major heat-shock genes is much
more complicated, and involves additional—or alternative—control channels.
FIGURE 1. Bacterial heat-shock response: regulation of operons coding for major chaperons.
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These regulatory elements will be reviewed looking at the groE and dnaK operons.
These operons, coding for the bacterial equivalent of Hsp10+60 and Hsp70,
respectively, contain in many bacteria a conserved regulatory inverted repeat
(IR=CIRCE), and are transcribed either by the vegetative sigma factor—σ70—or
by a σ32-like factor. The IR functions at the DNA level as a repressor binding site
and also controls the half life of the transcript. In addition, in Agrobacterium tumefaciens there also exists a system for mRNA processing that involves a temperature-controlled cleavage of the groE transcript.
ACKNOWLEDGMENTS
This work was supported in part by the Manja and Morris Leigh Chair for
Biophysics and Biotechnology.
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