Dual control for transcription of the galactose operon by cyclic AMP and its receptor protein at two interspersed promoters

Our results demonstrate the existence of two initiation sites, S 1 and S 2, for transcription of the galactose operon of E. coli. Transcription from each of these sites responds to different regulatory mechanisms. In the presence of Cyclic AMP Receptor Protein (CRP) and cyclic AMP (cAMP), transcription initiates only at S 1. In the absence of CRP or cAMP, transcription starts from S 2. Examination of a gal promoter mutation shows that transcription from S 1 is abolished, but that initiation can occur at S 2 and is still subject to repression by CRP-cAMP. In vivo gal expression from this mutant promoter is actually increased when cyclic AMP or CRP are eliminated by mutations in the cya (adenylate cyclase) or crp genes. Studies of gal expression from a wild-type promoter in cya and crp mutants are also consistent with the use of the two startpoints in vivo and suggest that when CRP or cAMP is absent, transcription starts from S 2 in vivo. S 1 corresponds to the startpoint for CRP- and cAMP-dependent gal mRNA previously determined (Musso et al., 1977). S 2 precedes S 1 by 5 base pairs in the DNA of the gal regulatory region. A heptamer sequence analogous to those described for other promoters precedes each startsite by a distance of 6 base pairs. DNA sequence analyses of the gal promoter mutation mentioned above establish that the base change is in the second residue of the heptamer preceding S 1.