Download Molecular genetics of bacteria

Genetic Recombination
• Bacteria are asexual
– With sexual reproduction, multiplication and gene
recombination are linked.
– In bacteria, they are separate
– Bacteria acquire new DNA from mutation, phage
infection, and transfer from other bacteria
• Bacterial genotypes are somewhat fluid
– Due to the ease of gene transfer, many genes can be
widely distributed among many bacteria
– One multicomponent organism?
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Gene transfer
• Ways that bacteria can acquire new genetic info
– Transformation
• Taking up of “naked DNA” from solution
– Transduction
• Transfer of DNA one to cell to another by a virus
– Conjugation
• “Mating”: transfer of DNA from one bacterium to
another by direct contact.
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Transformation
Both G+ and Gram –
bacteria can take up DNA.
Cells in a state in which
they can take up DNA are
referred to as competent.
DNA may be actively
released by some cells, http://openwetware.org/images/0/0c/Competence2.jpg
suggesting that DNA
exchange is “intentional”.
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Transformation requires homologous
recombination
New DNA must be similar.
Successful transformation requires that donor
and recipient be related.
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Transduction: the vector is a virus
• New DNA brought by a bacteriophage
– Requires donor and recipient be related
• Generalized transduction
– Chopped up bacterial DNA is incorporated into a capsid
– Phage binds to new host bacterium, but injects donor
bacterial DNA instead
• Specialized transduction
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In lysogeny, virus DNA inserts.
If DNA excises, can take adjacent bacterial DNA with it
Upon infection of new host, new bacterial DNA added
Important in bacterial evolution
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Gene transfer between bacteria-2
• Transduction: transfer of
DNA via a virus.
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Conjugation
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• A plasmid that can be
spread by conjugation
codes for a sex pilus.
• Pilus attaches donor to
recipient cell.
– Shortening of pilus draws
bacteria into contact
– Channel opens thru cell
wall of both bacteria
• Copy of plasmid sent.
http://parts.mit.edu/igem07/images/1/16/BU_conjugation.jpg
Conjugation and gene transfer
• Sex pili bind specifically to surface
molecules on bacteria
– But many bacteria have similar
molecules and can participate.
• Homologous recombination is
not needed
– Plasmids remain in cytoplasm
– Once the plasmid has been copied and sent, both donor
and recipient bacteria have the genes. Recipient can now
be a donor.
– Several types of plasmids, incl R, can be widely spread
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Regulation of genetic information
Bacteria are successful because
1. They share genetic information with other bacteria,
increasing their ability to adapt to their environment.
2. They carefully regulate their use of energy in metabolic
processes by shutting down unneeded pathways at the
biochemical and genetic levels.
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Bacteria tightly regulate their activities
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Bacteria must respond quickly to changes in the environment.
Bacteria are small compared to their environment, have no
real capacity for energy storage.
Simultaneous transcription and translation allows them to
synthesize the proteins they need quickly.
Wasteful activities are avoided. If there are sufficient amounts
of some metabolite, bacteria will avoid making more
AND avoid making the enzymes that make the
metabolite.
Biosynthesis costs!
Biochemical regulation and genetic regulation.
Biochemical regulation: allosteric enzymes
• Allo = other; steric = space. Many enzymes not only have
an active site, but an allosteric site.
• Binding of a molecule there causes a shape change in the
enzyme. This affects its function.
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Feedback inhibition of pathways
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Genetic regulation
• Genotype is not phenotype: bacteria possess many
genes that they are not using at any particular time.
• Transcription and translation are expensive; why
spend ATP to make an enzyme you don’t need?
• Examples:
– Induction of lactose operon
– diauxic growth with sugars.
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More on Regulation
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• In biochemical regulation, processes like feedback
inhibition prevent wasteful synthesis.
• To save more energy, bacteria prevent the synthesis of
unneeded enzymes by preventing transcription.
– In operons, several genes that are physically adjacent are
regulated together.
• Two important patterns of regulation: Induction and
repression.
– In induction, the genes are off until they are needed.
– In repression, the genes normally in use are shut off
when no longer needed.
Operons and Regulons
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• Nearly 50 years ago, Jacob and Monod proposed the operon
model.
• Many genes in prokaryotes are grouped together in the
DNA and are regulated as a unit. Genes are usually for
enzymes that function together in the same pathway.
• At the upstream end are sections of DNA that do not code,
but rather are binding sites for proteins involved in
regulation (turning genes on and off).
• The Promoter is the site on DNA recognized by RNA
polymerase as place to begin transcription.
• Operator is location where regulatory proteins bind.
• Promoter and Operator are defined by function.
Binding of small molecules to proteins causes
them to change shape
Characteristic of many DNA-binding proteins
Regulation of operons:
Inducible operons: Repressor protein comes off DNA
Repressible operons: Repressor protein attaches to DNA
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Binding of lac repressor to DNA
More lac Repressor Research - Lewis & Lu Labs
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Lac repressor
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Each different
colored ribbon
structure is a
polypeptide; the
repressor is a
tetramer and
binds both to
allolactose the
inducer and the
DNA.