Selective regulation of ptsG expression by Fis - Formation of either activating or repressing nucleoprotein complex in response to glucose

Transcription of ptsG encoding glucose-specific permease, enzyme IICBGlc, in Escherichia coli is initiated from two promoters, P1 and P2. ptsG transcription is repressed by Mlc, a glucose-inducible regulator of carbohydrate metabolism. The regulation of ptsG P1 transcription is also under positive control by cyclic AMP receptor protein and cyclic AMP complex (CRP-cAMP) as observed in other Mlc regulon. We report here that Fis, one of the nucleoid-associated proteins, plays a key role in glucose induction of Mlc regulon. ptsG transcription was induced when wild-type cells were grown in the presence of glucose. However, in a fts mutant, the basal level of ptsG transcription was higher but decreased when cells were grown in the presence of glucose, which implies the possibility of regulatory interactions among Fis, Mlc, and CRP-cAMP. Footprinting experiments with various probes and transcription assays revealed that Fis assists both Mlc repression and CRP-cAMP activation of ptsG P1 through the formation of Fis-CRP-Mlc or Fis-CRP nucleoprotein complexes at ptsG P1 promoter depending on the availability of glucose in the growth medium. ptsG P2 transcription was inhibited by Fis and Mlc. Tighter Mlc repression and enhanced CRP-cAMP activation of ptsG P1 by Fis enable cells to regulate Mlc regulon efficiently by selectively controlling the concentration of enzyme IICBGlc that modulates Mlc activity.