Transcriptional activation of nitrogen fixation genes in Klebsiella pneumoniae
The regulation of nif gene expression has been extensively studied in Klebsiella pneumoniae. It remains the model organism for studying many of the molecular mechanisms which underly the transcriptional control of nif in diazotrophs. The basic model for K. pneumoniae nif control is depicted in Figure 1. The rpoN product is a sigma factor (σ54) which complexes with core RNA polymerase (E) to form an RNA polymerase holoenzyme (Eσ54) which recognises the -12, -24 downstream promoter element (DPE) of nif promoters. The interaction of Eσ54 with the DPE is itself nonproductive with respect to the initiation of transcription, and activation of nif transcription requires the positive control proteins NtrC (which activates nifLA transcription) and NifA (which activates the other nif operons). The activity of NifA is controlled by NifL in response to nitrogen and oxygen status, that of NtrC by phosphorylation by NtrB in response to nitrogen status, ensuring expression of nif occurs only under nitrogen-limiting anaerobic or microaerobic conditions.
KeywordsIntegration Host Factor Promoter Complex Amino Terminal Domain Downstream Promoter Element Nitrogen Fixation Gene
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