The SOS repair genes in E. coli are negatively regulated by the lexA gene produc

Question

The SOS repair genes in E. coli are negatively regulated by the lexA gene product, called the LexA repressor. When a cell sustains extensive damage to its DNA, the LexA repressor is inactivated by the recA gene product (RecA), and transcription of the SOS genes is increased dramatically.

One of the SOS genes is the uvrA gene. You are a student studying the function of the uvrA gene product in DNA repair. You isolate a mutant strain that shows constitutive expression of the UvrA protein. Naming this mutant strain uvrAC , you construct the following diagram showing the lexA and uvrA operons.

Which two mutations would result in a uvrA constitutive phenotype, i.e. a phenotype in which the UvrA protein is expressed all the time? Indicate the actual genotypes involved.

Select the two correct answers.

mutation in uvrA- (mutation in the gene itself)

mutation in lexA- (mutation in the repressor gene product)

mutation in uvrAO- (mutation in the operator)

mutation   urvAP- (mutation in the promotor)

mutation in uvrA- (mutation in the gene itself)

mutation in lexA- (mutation in the repressor gene product)

mutation in uvrAO- (mutation in the operator)

mutation   urvAP- (mutation in the promotor)

Explanation / Answer

The two mutations which would result in a uvrA constitutive phenotype will be

b. mutation in lexA- (mutation in the repressor gene product) so that the SOS genes are not negatively repressed.

c. mutation in uvrAO- (mutation in the operator) because an operator is a segment of DNA to which a transcription factor binds to regulate gene expression by repressing it. Thus, mutating it will lead to the non-binding of TF which can repress the action of UvrA enabling it to be constitutive.

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