nif gene expression studies in Rhodobacter capsulatus: ntrC-independent repression by high ammonium concentrations.

The expression of nif genes in Rhodobacter capsulatus depends on the two regulatory genes, rpoN and nifA, encoding a nif-specific alternative sigma factor of RNA polymerase and a nif-specific transcriptional activator, respectively. The expression of the rpoN gene itself is also RPON/NIFA dependent. In order to better characterize the regulation of nif gene induction, chromosomal nifH-, rpoN-, nifA1- and nifA2- lacZ fusions were constructed and the expression of these different nif-lacZ fusions was determined under photoheterotrophic conditions at different starting ammonium concentrations. The two nifA genes were found to be induced first, followed by nifH and finally by rpoN upon weak, medium and strong nitrogen starvation, respectively. This induction profile and the correlation between the expression of the different nif genes suggested that nifA1 expression is the limiting factor for nif gene induction. This hypothesis was tested by construction of different nifA1 overexpressing mutants. Contrary to the current model of nif gene expression in R. capsulatus, which predicted constitutive nif gene expression in such mutants, a strong repression of nifH and rpoN was found at high ammonium concentration. The low nifH expression under these conditions is unaffected by nifA2 and is not increased in a ntrC mutant, ruling out any role of NTRC as a mediator of this repression. This finding implies an additional, so far unidentified, regulation by fixed nitrogen in R. capsulatus. Changing the expression level of rpoN indicated that low levels of RPON are already sufficient for full nifH induction. The nifA1 and rpoN expression mutants were also tested for diazotrophic growth. Similar generation times were determined for the mutants and for the wild type, but diazotrophic growth of the nifA1 over-expressing ntrC mutant RCM14 did not start until after a prolonged lag phase of two to three days.