REPRESSION OF THE KLEBSIELLA-AEROGENES NAC PROMOTER

Transcription of the nitrogen-regulated nac promoter of Klebsiella aerogenes requires sigma(54) RNA polymerase, is activated by the phosphorylated form of the transcription factor nitrogen regulator I (NRI) (NtrC), and is repressed by the product of the nac gene, Nac, Nac protects a large portion of the nac control region, extending from positions -130 to -70, from digestion by DNase I, This site(s) lies immediately upstream from the site at which sigma(54) RNA polymerase binds, is downstream of a high-affinity binding site for the transcriptional activator NRI similar to P, and partially overlaps a low-affinity NRI similar to P-binding site, Binding of Nac to the DNA resulted in bending of the DNA but did not interfere with the binding of sigma(54) RNA polymerase to the promoter or with the binding of NRI similar to P to either the high-affinity site or low-affinity site. Furthermore, transcription assays with various wild-type and mutant templates suggested that Nac did not exclude NRI similar to P from either the low- or high-affinity sites, nor did Nac interfere with the ability of the polymerase to form the open complex when the binding sites for NRI similar to P were moved to different locations upstream from the promoter, Rather, Nac seemed to repress by an antiactivation mechanism in which the interaction of the NRI similar to P, bound at its normal sites, with sigma(54) RNA polymerase, bound to the promoter, was prevented.