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 DNA the LexA repressor 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, construct the following diagram showing 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. a- mutation in uvrA- (mutation in the gene itself) b. mutation in lexA- (mutation in the repressor gene product) c. mutation in uvrA degree. (mutation in the operator) d. mutation urvA^P- (mutation in the promoter)

Explanation / Answer

Answer: (b) mutation in lexA- (mutation in the repressor gene product), and (c) mutation in uvrAO- (mutation in the operator)

Explanation:

Mutations that inactivated lexA should lead to a constitutive uvrA phenotype as the uvrA gene will no longer be repressed.

Mutations in the uvrA operator should also cause a constitutive uvrA phenotype as they would block the binding of LexA.

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