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Transcription initiation inArchaea (archaebacteria) resembles the eucaryotic process, having been shown to involve TATA box-like promoter regions as well as TATA-binding protein and TFIIB homologs. However, little is known about transcription regulation in archaea. We have previously demonstrated that transcripts ofnifHDK2 genes, encodingMethanosarcina barkeri nitrogenase, are present in N2 -grown cells but not in ammonium-grown cells, indicating thatnif transcription is regulated by the nitrogen source. In this study, we detected proteins inM. barkeri cell extracts that bind specifically to DNA containing the putative promoter region ofnifHDK2 . No binding was found when the promoter region was deleted from the DNA. A competition assay showed that the methyl coenzyme M reductase (mcr ) promoter region DNA and thenifH2 promoter region DNA competed for a common factor(s). There was no binding to thenifH2 promoter region by extracts of ammonium-grown cells, but there was binding by these extracts to promoter regions formcr genes, which are presumably constitutively expressed. Interestingly, extracts of ammonium-grown cells inhibited binding to thenif promoter region by extracts of N2 -grown cells. Fractionation of extracts of ammonium-grown cells with a heparin-Sepharose column resolved them into a fraction eluting at 0 M NaCl, which inhibited binding by extracts of N2 -grown cells, and a fraction eluting at 0.5 to 0.75 M NaCl, which showed binding to the promoter region. These results are congruent with a model for regulation ofnif gene expression inM. barkeri in which a substance present in ammonium-grown cells inhibits DNA binding by a transcription-associated protein or proteins.

journal of bacteriology

Interactions between the Promoter Regions of Nitrogenase Structural Genes ( nifHDK2 ) and DNA-Binding Proteins from N 2 - and Ammonium-Grown Cells of the Archaeon Methanosarcina barkeri 227