Slippage synthesis at the galP2 promoter of Escherichia coli and its regulation by UTP concentration and cAMP.cAMP receptor protein.

An intriguing mechanism in regulating transcription initiation from the gal operon in Escherichia coli is described. Initiation from galP2, one of the two promoters of the E. coli galactose operon, is shown to be subject to promoter clearance control in responding to changes in UTP concentration. In vitro, RNA polymerase (RNAP) makes a large amount of nonproductive "stuttering" initiation products at the galP2 promoter at high concentrations of UTP and less of the stuttered products at low concentrations of UTP. Conversely, RNAP makes more productive initiation products at low UTP concentration than at high UTP concentration. The transcription factor cAMP.CRP complex which normally inhibits transcription from galP2 also represses the stuttering synthesis from galP2. When galactose is used as a sole carbon source and the internal UTP pools are adjusted externally, a cya mutant (in which galP2 is mainly responsible for the expression of the gal operon and galP1 activity is minimal) has a slower growth rate and lower expression of the gal operon at high UTP pools than at low UTP pools. Such an apparent correlation between the in vitro and in vivo results allows one to speculate that changes in UTP concentration can modulate the expression of the gal operon. The implication of a gal promoter being controlled by UTP is discussed.