SHAPE * MERGEFORMAT

A special class of mutants of E coli unable to utilize lactose as a carbon source were found to have mutations in the coding sequence for ?-galactosidase (lacZ gene). They produce no ?-galactosidase protein. Surprisingly, they fail to produce the polypeptide products of the permease (lacY) and the transacetylase (lacA). In addition, no mRNA from any of these genes is detected.

Three kinds of revertants were isolated:

Type I Revertants: Regained permease (LacY) and transacetylase (LacA) activities but not ?-galactosidase activity. These reverting mutations were found within the lac operon.

Type II Revertants: Regained all 3 enzymatic activities.

Type III Revertants: Regained only permease and transacetylase activities by a secondary mutation that mapped in the gene for rho factor.

Lac operon DNA was isolated from the original mutant strain. The strands of the DNA were separated from each other and mixed with single strands of DNA containing the lac operon isolated from a wild type bacterium. These “mixed” DNAs were allowed to hybridize to each other.

The resulting double helix structures were isolated and visualized via electron microscopy:

SHAPE * MERGEFORMAT

A. What is a revertant mutation (in general)?

B. What was the genetic event that caused the original lac- mutation (ie did not allow for the expression of lacZ, lacY, or lacA)?

C. Why did this event result in failure to produce permease and transacetylase?

D. What is the nature of the Type I Reverting mutations?

E. What is the nature of the Type II Reverting mutations?

F. What is the nature of the Type III Reverting mutations?