Regulation of Molybdenum and Alternative Nitrogenases in the Photosynthetic Purple Bacterium Rhodobacter Capsulatus

Abstract

The photosynthetic nonsulfur purple bacterium Rhodobacter capsulatus is able to fix dinitrogen (N2) using either a conventional molybdenum nitrogenase or an alternative, heterometal-free nitrogenase. Until now, thirtyfive genes have been characterized in detail that are involved in synthesis and regulation of the molybdenum nitrogenase (Klipp, 1990; Schmehl et al., 1993). These genes are localized in three distinct regions of the R. capsulatus chromosome, designated nif regions A, B and C. Expression of nif operons is controlled by a regulatory circuit similar to the Ntr-system of enteric bacteria (Fig. I). R. capsulatus nifRI, nifR2 and nifR4 are homologous to ntrC, ntrB and rpoN, respectively. Under conditions of nitrogen limitation this nif specific Ntr-system activates two functional copies of nifA, which in tum activate all other nitrogen fixation genes. However, in contrast to the Ntr system of enteric bacteria, no genes involved in the general nitrogen metabolism are activated by R. capsulatus NifRl. Mapping of transcriptional startpoints of nifAf and nifAII revealed a proximal-23/-7 promoter element (Foster-Hartnett, Kranz, 1992; Preker et aI., 1992) which might be the recognition site of an RNA polymerase that contains an as yet unidentified sigma factor (,.,x in Fig. I). Transcriptional activity of this polymerase strictly depends on binding of the phosphorylated fonn of NtrC to tandemly arranged distal DNA elements (Foster-Hartnett et aI., 1994). The gene products of nifAI and nifAII belong to the group of oxygen-sensitive NifA proteins and act as transcriptional activators for the expression of all other nif genes together with RNA polymerase containing (754. This sigma factor is encoded by nifR4 and transcriptionally controlled by an autoregulatory circuit consisting of NifA and NifR4 itself (Cullen et aI., 1994).