Are all lac- stains the same?

At this point, we do not know there there is only a single gene which, when mutated, leaves cells lac- or whether there are multiple genes that can be mutated to produce a lac- phenotype.

Since we know about ß-galactosidase, an obvious first question is do the lac- strains make ß-galactosidase in response to lactose?

Are any of the lac- strains able to make ß-galactosidase in the presence of IPTG?

Those that do presumably have a normal forms of the ß-galactosidase gene and all of the steps require

• I thought that the presence of glucose suppressed lactose ulitilization and ß-galactosidase expression -- why did we use glucose containing plates?

In the course of their work, Monod et al identified a second lactose inducible gene - lactose permease.

We can assay for the presence of active permease activity by incubating the cells with a radioactive form of galactoside. Only if the permease is active will the radioactive molecules be accumulated within the cells.

We now have three possible lac- stains.

• lacR: do not make either ß-galactosidase or lactose permease in the presence of lactose or IPTG.
• lacY: make ß-galatosidase, but not lactose permease in the presence of lactose or IPTG.
• lacZ: make lactose permease, but not ß-galactosidase in the presence of lactose or IPTG.

It is likely that lacR mutations are due to the presence of a single mutation, although we could image the phenotype is due to two or more independent mutations.

lacY mutations are likely to be in the lactose permease, but not in the gene(s) involved in the lactose induced gene expression or the ß-galactosidase gene.

lacZ mutations are likely to be in the ß-galactosidase gene, but not in the gene(s) involved in the lactose induced gene expression or the lactose permease gene.

revised 9 July 2003