Molecular Genetics of Prokaryotes Problem Set

Problem 9: Phenotype of lac I- cells

Tutorial to help answer the question

An E. coli strain is lac I. The lac I DNA sequence encodes the lactose repressor. How would you describe the regulation of lactose metabolism in these cells?


Structure of lac repressor protein bound to lac operator

The lac I gene is the regulatory gene of the lac operon. The lac I gene encodes the lac repressor protein. When lac repressor protein is bound to the lac operator DNA sequence, mRNA synthesis from the structural genes of the operon is blocked. In a lac I mutant, active repressor protein is not synthesized. As a result, mRNA synthesis from the lac operon is constitutive, and there is constitutive expression of the lac Z gene product, ß-galactosidase.

The structure of the complex formed between the lac repressor protein DNA binding domain and the lac operator DNA sequence is shown in the figure:

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