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01:119:115 Lecture 16: Gene Regulation

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School
Rutgers University
Department
Biological Science
Course
01:119:115
Professor
Beal

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Document Summary

Only a certain subset of genes are used at any given time. Other genes are only transcribed when needed or at a certain time. Glucose is a better source of energy than lactose. Normally, lac operon is blocked, rna polymerase binds to promoter but cannot transcribe. In prokaryotes, genes are regulated at transcription (when it needs to) 3 genes for lactose metabolism in e. coli on dna. If no lactose = cell does not make enzymes. If lactose is present = enzymes are made. Operon = a group of genes controlled by a single regulatory signal. All genes work together (transcribed or not at all) Structure genes (lac y, lac z, lac a) Promoter and operator are binding sites on dna = not transcribed. Usually off transcription needs to be turned on. To turn on, an inducer molecule inactivates the repressor. Repressor prevents transcription (repressor is a constitutive gene) Allolactose (inducer) binds to repressor => change its shape!

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Related Questions

1. The lac operon of E. coli controls the expression of genes that code for enzymes involved in lactose metabolism. The operon system is inducible in that it can be ‘turned on’ by the presence of lactose. Mutations to various regions of the lac operon have been discovered which affect both the control of gene expression and the function of the gene products.

These mutations include:

I - = produces a repressor protein that can not bind to the operator region

I s = produces a repressor protein that can not be removed from the operator region

O C = can not bind a repressor protein

Z - = produces a defective (non-functioning) β-galactosidase enzyme

( I- are recessive to I+; I+ recessive to IS )

Detail the functionality of the lac

operon under the following genetic and cellular conditions.

GENES

Lactose

present

Repressed

(yes/no)

Constitutive

(yes/no)

Inducible

(yes/no)

I + O + Z +

no

I + O C Z +

no

I S O + Z +

yes

I - O + Z +

yes

I S O C Z +

no

A theoretical operon (amo) in E. coli contains several structural genes encoding enzymes involved in the biosynthesis of a particular amino acid. Unlike the lac operon, in which the repressor gene is separate from the operon itself, for the amo operon, the repressor gene is found within the operon itself. When the end product amino acid is present, it binds to the repressor, and this complex binds to the operator, repressing the operon. In the absence of the amino acid, the repressor fails to bind to the operator, and transcription proceeds. Characterize this operon, comparing it to the regulation of the lac and trp operons. Be sure to discuss how regulation is affected by having the repressor gene as part of the operon.

Then, consider the following mutations:

a) Mutation in the operator region;

b) Mutation in the promoter region;

c) Mutation in the repressor gene.

In each case, state whether the operon will be transcribed or not, and describe how the mutation affects the regulation of the operon. Compare each response to the equivalent situation in the lac and trp operons.

In E.coli the lac operon is controlled by the presence or absence of both lactose and glucose. The lac repressor only allows expression in the presence of lactose, and it is also only activated when glucose levels are low, such that the lac operon is on when lactose is present but glucose is not.

You have cells that are mutant in the gene coding for the Lac repressor (I-). These cells lack the Lac repressor under all conditions. For these mutant cells, state whether the Lac operon will be switched on or off in the following situations, whether the lac repressor (I) is bound to the operator, and whether CAP-cAMP is bound upstream of the promoter.

a. in the presence of glucose and lactose

b. in the presence of glucose and the absence of lactose

c. in the absence of glucose and the absence of lactose

d. in the absence of glucose and the presence of lactose