NIFA-DEPENDENT INVIVO PROTECTION DEMONSTRATES THAT THE UPSTREAM ACTIVATOR SEQUENCE OF NIF PROMOTERS IS A PROTEIN-BINDING SITE

Primer-extension analysis of the Klebsiella pneumoniae nifH promoter was used to determine changes in the accessibility of the promoter DNA to methylation after exposure of growing cells to dimethyl sulfate. Four guanine residues present in the nifH upstream activator sequence (UAS), the proposed NifA binding site, were protected from methylation and two guanine residues were hypermethylated when the transcriptional activator protein NifA was present in the cells. The interaction detected at the nifH UAS was independent of the alternative .omega. factor NtrA required for transcription of the nifH and other nif promoters. Mutations within the nifH UAS that diminish NifA-depednent transcriptional activation reduced the interaction at the UAS. It seems likely that the pattern of methylation protection observed in the nifH UAS is the result of NifA binding.