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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2Gene 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.

Transformation 3

http://openwetware.org/images/0/0c/Competence2.jpg

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, suggesting that DNA exchange is “intentional”.

Transformation requires homologous recombination

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New DNA must be similar.

Successful transformation requires that donor and recipient be related.

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– 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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6Gene transfer between bacteria-2

• Transduction: transfer of DNA via a virus.

Conjugation

• 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.

7

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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9Regulation 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.

10Bacteria tightly regulate their activities

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.

11Biochemical 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.

12Feedback inhibition of pathways

13Genetic 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.

14More on Regulation

• 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.

15Operons and Regulons

• 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.

16Binding 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 DNARepressible operons: Repressor protein attaches to DNA

Binding of lac repressor to DNA 17

More lac Repressor Research - Lewis & Lu Labs

Lac repressor 18

Each different colored ribbon structure is a polypeptide; the repressor is a tetramer and binds both to allolactose the inducer and the DNA.