Genetic regulation
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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? Operon Genes physically adjacent regulated together Regulon

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Genetic regulation

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Genetic regulation

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?

  • Operon

    • Genes physically adjacent regulated together

  • Regulon

    • Genes dispersed but controlled by same proteins

    • Operator sequences must be same/similar


More on regulation

More on Regulation

  • 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.

  • Negative control

    • Binding of protein to the DNA prevents transcription

  • Positive control

    • Binding of protein to DNA promotes transcription


Repressible operons

Repressible operons

  • Operon codes for enzymes that make a needed amino acid (for example); genes are “on”.

    • Repressor protein is NOT attached to DNA

    • Transcription of genes for enzymes needed to make amino acid is occurring.

  • The change: amino acid is now available in the culture medium. Enzymes normally needed for making it are no longer needed.

    • Amino acid, now abundant in cell, binds to repressor protein which changes shape, causing it to BIND to operator region of DNA. Transcription is stopped.

  • This is also Negative regulation (protein + DNA = off).


Repression picture

Repression picture

Transcription by RNA polymerase prevented.


Regulation can be fine tuned

Regulation can be fine tuned

The more of the amino acid present in the cell, the more repressor-amino acid complex is formed; the more likely that transcription will be prevented.


Structure of the lac operon

Structure of the Lac operon

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.


Binding of small molecules to proteins causes them to change shape

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 DNARepressible operons: Repressor protein attaches to DNA


How the lac operon works

How the lac operon works

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

Lactose is not actually the inducer

Low basal levels of beta-galactosidase 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

Glucose is the preferred carbon source


Positive regulation

Positive regulation

  • 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/.../catabolite-activator-protein


Additional controls

Additional controls

  • Attenuation

    • Seen w/ repressible operons, fine tuning

    • Ribosome does not stall, transcription terminated

  • mRNA rapidly degraded

    • Signal “to make” stops, residual mRNA destroyed

  • Examples of

    • Antisense RNA: binds to mRNA, prevents use

    • DNA rearrangements; genes flip in place, different gene product produced

    • Ribosome binding protein prevents translation


Global control modulons

Global control: modulons

  • Different operons/regulons affected by same environmental signal

    • Presence of glucose

    • Change from O2 to anaerobic growth

    • Nitrogen limitation; phosphate starvation

    • Growth rate control

    • Cell division

    • Stationary phase; entering starvation state

  • One method of control: alternate sigma factors

    • Sigma controls which promoters are used


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