Working out how a cell responds to events is incredibly difficult. In general, cells change the expression of genes in response to signals such as how much food there is or changes in temperature. But the effects on gene expression are complex and, without better models, difficult to predict.
Working at the Weizmann Institute of Science, Shai Kaplan, Anat Bren and others worked out a way of mapping how a set of 20 genes in e coli cells react to changes in sugar levels. E coli is able to use several different sugars as sources of carbon:
"Each sugar system includes transporters that pump the sugar into the cell and enzymes that break it down. Each system also includes a transcription factor that senses the presence of the sugar and accordingly regulates gene expression. In addition, most of the sugar systems are regulated by a master transcription factor called CRP, which senses the starvation of the cell. This master regulator is activated by cAMP, a small molecule produced in the cell upon glucose starvation."
In principle, the sugar genes in the regulatory network should behave in a similar way: their expression should increase with the availability of a given sugar and, according to conventional wisdom, glucose starvation. So, it should be possible to model the system as a set of logic AND gates. But, one study of the lactose-input system showed a more complex logic function: a mixture of an AND and an OR.
