fixK, a gene homologous with fnr and crp from Escherichia coli, regulates nitrogen fixation genes both positively and negatively in Rhizobium meliloti.

Abstract

Nitrogen fixation genes are shown to undergo a complex positive and negative regulation in Rhizobium meliloti. Activation of fixN by fixLJ is shown to require a third regulatory gene, fixK. As fixK is activated by fixLJ, we propose a cascade model for fixN regulation such that fixLJ activates fixN via fixK. In addition fixK negatively regulates expression of the nif‐specific activator nifA as well as its own expression by autoregulation. Thus nifA and fixK are subject to a mixed regulation, positive (by fixLJ) and negative (by fixK). The sequence of fixK shows homology with the Escherichia coli regulators fnr and crp, which makes fixK the third characterized member of this family of prokaryotic regulators.

Nucleotide sequence and predicted functions of the entire Sinorhizobium meliloti pSymA megaplasmid

“…meliloti FixK1 and FixK2 are Fnr͞Crp-like transcription factors that positively control expression of fixNOQP and negatively control expression of nifA (44,45). We identified a third FixK-like protein (SMA0662).…”

Symbiotic Nitrogen Fixation

“…In S. meliloti, expression of fixNOQP is regulated mainly through an O 2 -sensing cascade comprised of the fixL and fixJ gene products; this cascade activates expression of the fixK gene, which leads to expression of the fixNOQP operon (2,3,9,12,18). fixK encodes a transcriptional activator belonging to the cyclic AMP receptor protein (CRP)-Fnr family (2).…”

“…fixK encodes a transcriptional activator belonging to the cyclic AMP receptor protein (CRP)-Fnr family (2). Interesting variations of this basic regulatory scheme have been found in other rhizobial strains.…”

“…viciae VF39 contains FixK (30) and another transcriptional activator of the CRP-Fnr family, FnrN which activates the two fixNOQP copies (8,15,16,30). In contrast to FixK, FnrN has a region with a high level of similarity to a domain in the Escherichia coli Fnr protein involved in O 2 sensing, suggesting that the FnrN transcriptional activity may be negatively modulated by O 2 (8). Both FixK and FnrN activate fixNOQP expression by binding to a DNA sequence located in the promoter region (TTGAT-N 4 -ATCAA) called the anaerobox (2,8,11).…”

“…In contrast to FixK, FnrN has a region with a high level of similarity to a domain in the Escherichia coli Fnr protein involved in O 2 sensing, suggesting that the FnrN transcriptional activity may be negatively modulated by O 2 (8). Both FixK and FnrN activate fixNOQP expression by binding to a DNA sequence located in the promoter region (TTGAT-N 4 -ATCAA) called the anaerobox (2,8,11). Furthermore, R. leguminosarum fnrN is able to complement an S. meliloti fixK mutant for fixNOQP expression (8,16), suggesting that the two proteins activate transcription in similar ways (8,15).…”