Defining the steps in adaptation to lactose |
|
ß-galactosidase activity appears in E. coli cells only after the addition of lactose. So the question arises, does lactose activate pre-existing ß-galactosidase or does it induce its de novo (new) synthesis?
When cleaved by ß-galactosidase ONPG produces an intensely yellow compound while X-GAL produces an intensely blue one. It is possible to measure ß-galactosidase activity by measuring the appearance of the colored product |
It is worth remembering that the ONPG and X-GAL assays measure enzyme activity, not the total amount of ß-galactosidase protein present. Typically, to measure protein levels we would use other (typically antibody-based) method. Cohn and Monod, produced antibodies specific for ß-galactosidase. This enabled them to follow both enzyme activity and protein levels. They found that the appearance of ß-galactosidase activity correlated with the appearance of ß-galactosidase protein. |
In order to understand how E. coli adapt to the presence of lactose, we will perform a genetic analysis. We will search for mutant cells that either fail to grow on lactose as their sole carbon/energy source or which express ß-galactosidase all of the time, i.e. constitutively. |
|
revised 9 July 2003 |