In the absence of glucose cell can make use of other sources of carbohydrates such as lactose. In E. Coli and other bacteria utilization of lactose depends on the availability of cAMP. Bacteria accumulate cAMP only when starved. When glucose is exhausted, cAMP is available in plenty.
cAMP is necessary for the gene expression because it binds catabolite repressor protein (CAP) to form CAP-cAMP complex. CAP is a lac repressor the binds to the operator site of the lac operon preventing RNAP From binding to the promoter.
RNA polymerase can bind to the promoter site only when CAP-cAMP formed
