Assembling the lac operon

The types of rob- mutations

Our genetic analysis indicates that rob- mutations map to lacI and lacO.

The lacI gene, encodes the lac repressor. O stands for "operator" and corresponds to the sequence of DNA to which the lac repressor binds.

When lac repressor is bound to O, expression of both lactose permease (lacY) and ß-galactosidase (lacZ) is inhibited.

A rob- phenotype occurs if lacI is mutated such that the lac repressor can no longer bind to the operator OR when there is a mutation in lacO so that it no longer can bind to the lacI protein.

The types of lac- mutations

Lac- mutations map to four different positions along the chromosome.

The most common are mutations that inactivate either the ß-galactosidase (lacZ) or lactose permease (lacY) genes, but not both.

There are also mutations that block the synthesis of both gene products. These map to a small region known as the promoter (lacP).

Finally, there are mutations that map to lacI. These mutations leave the lac repressor able to bind to the operator, but abolishes its ability to bind to lactose affectors (e.g. IPTG and allolactone).

So even in the presence of lactose, the repressor is still repressing.

Take a strain with a mutation in lacI that is lac-. Now generate new mutations that make this strain rob-. Where would you expect such mutations to map?

revised 9 July 2003