Activation of the bgl operon by adaptive mutation

In growing Escherichia coli K12 cells, the cryptic bgl operon is activated98% of the time by insertions of IS1 or IS5 into the control region,designated bglR. The activated bgl operon permits utilization of thebeta-glucoside sugar arbutin as a sole carbon and energy source. The bgloperon is also activated by late-occurring mutations during prolongedselection on arbutin. The late-occurring mutations that occurred duringprolonged carbon starvation in the presence of arbutin were "adaptivemutations" because they were specific to the presence of arbutin, and theydid not occur during prolonged starvation in the absence of arbutin. Thespectrum of late-arising mutations differed from that of early-arising,growth-dependent mutations in that 20% of the late-arising mutants resultedfrom mutations at the hns locus. This provides the first direct evidencefor adaptive mutagenesis mediated by the insertion of IS elements. Becauseno special genetic background is required to select Bgl+ mutants, thisaffords the opportunity to study IS-element-mediated adaptive mutagenesisin a variety of genetic backgrounds, including the backgrounds of naturalisolates of E. coli.