Transcript Molecular genetics of bacteria

Molecular genetics of bacteria
• Gene regulation and regulation of metabolism
• Genetic exchange among bacteria
Bacteria are successful because
1. They carefully regulate their use of energy in metabolic
processes by shutting down unneeded pathways at the
biochemical and genetic levels.
2. They share genetic information with other bacteria,
increasing their ability to adapt to their environment.
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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.
Our example: the lac operon
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• Lactose is milk sugar, used by a few bacteria like E. coli
• To use lactose, a couple of proteins are important: the
permease which transports the sugar into the cell, and the
enzyme beta-galactosidase which breaks the disaccharide
lactose into glucose and galactose.
• To prevent the expense of synthesizing these enzymes if
there is no lactose to use, E. coli keeps these genes inactive.
But if lactose becomes available, these genes must be turned
on quickly so lactose can be taken in and broken down.
– Imagine E. coli in your GI tract at breakfast.
Structure of the Lac operon
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KEY:
P O are the promoter
and operator regions.
lac Z is the gene for
beta-galactosidase.
lac Y is the gene for
the permease.
lac A is the gene for
a transacetylase.
lac I, on a different
part of the DNA, codes
for the lac repressor,
the protein which can
bind to the operator.
Products of the lac operon
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• Each gene codes for a protein that is involved in the use of
lactose. Depicted is the mRNA and the proteins that result.
Note that P and O are functional regions of DNA but aren’t
genes for proteins; no mRNA is made from them.
http://www.med.sc.edu:85/mayer/genreg1.jpg
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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How the lac operon works
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When lactose is NOT present,
the cell does not need the
enzymes. The lac repressor,
a protein coded for by the
lac I gene, binds to the DNA
at the operator, preventing
transcription.
When lactose is present, and
the enzymes for using it are
needed, lactose binds to the
repressor protein, causing it
to change shape and come off
the operator, allowing RNA
polymerase to find the
promoter and transcribe.
http://www.med.sc.edu:85/mayer/genreg1.jpg
Lactose is not actually the inducer
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Low basal levels of betagalactosidase exist in the
cell. This converts some
lactose to the related
allolactose which binds to
the lac repressor protein.
Synthetic inducers such as
IPTG with a similar
structure can take the place
of lactose/allolactose for
research purposes.
http://www.search.com/reference/Lac_operon
Glucose is the preferred carbon source
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Positive regulation
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• Presence of lactose is not enough
– In diauxic growth graph, lactose is present from the start.
Why isn’t operon induced?
• Presence of glucose prevents positive regulation
– NOT the same as inhibiting
– Active Cyclic AMP receptor protein (CRP) needed to
bind to DNA to turn ON lactose operon (and others)
– Presence of glucose (preferred carbon source) prevents
activation of CRP.
www.answers.com/..
./cataboliteactivator-protein
Plasmids
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• Plasmids: small, circular,
independently replicating pieces of
DNA with useful, not essential info
• Types of plasmids
– Fertility,
– resistance,
– catabolic,
– bacteriocin,
– virulence,
– tumor-inducing, and
– cryptic
http://www.estrellamountain.edu/faculty/farabee/biobk/14_1.jpg
About plasmids-1
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Fertility plasmid: genes to make
a sex pilus; replicates, and a copy
is passed to another cell.
Resistance plasmid: genes that
make the cell resistant to
antibiotics, heavy metals.
Catabolic plasmid: example, tol
plasmid with genes for breaking
down and using toluene, an
organic solvent.
www.science.siu.edu/.../ micr302/transfer.html
About plasmids-2
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• Bacteriocin plasmid: codes for bacteriocins, proteins that
kill related bacteria.
• Virulence plasmid: has genes needed for the bacterium to
infect the host.
• Tumor-inducing plasmid: The Ti plasmid found in
Agrobacterium tumefaciens. Codes for plant growth
hormones. When the bacterium infects the plant cell, the
plasmid is passed to the plant cell and the genes are
expressed, causing local overgrowth of plant tissue = gall.
Very useful plasmid for cloning genes into plants.
• Cryptic: who knows?
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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Gene transfer between bacteria
• Transformation: uptake
of “naked” DNA from
medium.
• When Griffith did his
experiment combining
heat killed, virulent cells
with live, harmless
mutants, the living cells
took up the DNA from
solution, changed into
capsule-producing,
disease-causing bacteria.
• Next slide
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Transformation details
DNA must be homologous, so transformation
only occurs between a few, close relatives.
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Gene transfer between bacteria-2
• Transduction: transfer of
DNA via a virus.
More common,
but still requires
close relative.
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Conjugation: bacterial sex
• If sex is the exchange of genetic
material, this is as close as bacteria
get. Conjugation is widespread and
does NOT require bacteria to be
closely related.
• Bacteria attach by means of a sex
pilus, hold each other close, and
DNA is transferred.
• Plasmids other than F plasmids, such
as resistance plasmids, can also be
exchanged, leading to antibioticresistant bacteria.
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