REPRESSOR PROPERTIES OF THE NIFL GENE-PRODUCT IN KLEBSIELLA-PNEUMONIAE

Certain mutations in the nifL gene of the K. pneumoniae nitrogen fixation (nif) gene cluster resulted in altered nif regulation such that nitrogenase synthesis was no longer repressed by low levels of exogenous fixed nitrogen, by O2 or by high temperature. Introduction of a plasmid with a nifL+ allele restored fixed nitrogen and O2 repression. The nifL product apparently acts as a nif-specific repressor in response to these effectors. Hence nif-specific regulation is controlled by the products of 2 regulatory genes, nifLA, which comprise a single operon. As previously reported (Dixon et al. 1980; Buchanan-Wollaston et al. 1981), the nifA product is necessary for transcription from other nif promoters but not from that of its own operon. The nifL gene product also acts at other nif promoters but does not repress its own synthesis. Transcription of nifLA is in turn controlled by a general nitrogen-regulatory system in the cell, mediated by the products of the ntrA (glnF), ntrB (glnL) and ntrC (glnG) genes.