Escherichia coli cells deleted for the cyclic AMP (cAMP) receptor protein (Crp) gene (Δcrp) cannot utilize glycerol because cAMP-Crp is a required positive activator of glycerol utilization operon glpFK. We have previously shown that a transposon, Insertion Sequence 5 (IS5), can reversibly insert into the upstream regulatory region of the operon so as to activate glpFK and enable glycerol utilization. GlpR, which represses glpFK transcription, binds to the glpFK upstream region near the site of IS5 insertion, and prevents insertion. We here show that the cAMP-Crp complex, which also binds to the glpFK upstream regulatory region, also inhibits IS5 hopping into the activating site. This finding allowed us to identify conditions under which wild type cells can acquire glpFK-activating IS5 insertions. Maximal rates of IS5 insertion into the activating site require the presence of glycerol as well as a non-metabolizable sugar analogue that lowers cytoplasmic cAMP concentrations. Under these conditions, IS5 insertional mutants accumulate and outcompete the wild type cells. Because of the widespread distribution of glucose analogues in nature, this mechanism of gene activation could have evolved by natural selection.