Chp 12

Negative control of transcription: involves repressor proteins binding to DNA sequences, inhibiting gene transcription (turns off)

Positive control of transcription: involves activator proteins binding to DNA sequences, promoting gene transcription (turns on)

Repressor proteins: regulatory proteins that exert negative control of transcription

When a repressor protein binds to an operator, it physically obstructs RNA polymerase from transcribing the gene.

Only happens when repressor protein is active!

Repressor proteins have two active sites: DNA-binding domain and allosteric domain

DNA-binding domain: a specific protein region that directly interacts with DNA to bind to a specific sequence

Allosteric domain: a site on a protein that, when bound by a molecule, can induce conformational changes in another part of the protein, indirectly affecting its function (best target for preventing the binding of a corepressor)

Repressible operon: a group of genes that are usually on, but can be turned off in response to environmental conditions.

Attenuation: is a regulatory mechanism that can prevent the transcription of certain genes in response to environmental conditions.

Repressors and corepressors can prevent transcription of a repressible operon.

lac operon structural genes

1. lacZ: encodes beta-galactosidase, which breaks down lactose into glucose and galactose.

2. lacY: encodes lactose permease, a protein that transports lactose into the cell

3. lacA: encodes beta-galactoside transacetylase, an enzyme with a less critical role in lactose metabolism

lac operon function

When lactose is absent, the lac repressor (produced by the lacI gene) binds to the operator and prevents transcription of the operon, meaning the genes for lactose utilization aren’t expressed.

When lactose is present, lactose molecules (or allolactose, a derivative) act as an inducer. They bind to the lac repressor, causing a conformational change that releases the repressor from the operator region. This allows RNA polymerase to bind to the promoter and initiate transcription of the lac genes, enabling the bacteria to produce the enzymes necessary for lactose metabolism (example of negative control)

The lac operon allows bacteria to conserve energy by only producing the enzymes required to metabolize lactose when it is present.

The genotype of a partial diploid can be written as:

F' I⁺ P⁺ O⁺ Z⁺ Y^- / I⁺ P⁺ O⁺ Z^- Y⁺

• + sign indicates that particular gene is wild type and functional

• - sign indicated that particular gene is mutant and lack of function

The genotype above cannot produce a functional permease (lacY^-) and the other copy is unable to produce a functional beta-galactosidase (lacZ^-)

tryptophan synthesis

3-4 stem loop of mRNA is the termination stem loop

2-3 stem loop of mRNA is the antitermination stem loop

1-2 stem loop of mRNA can induce a pause in the attenuation