The four different σ⁵⁴ factors of Rhodobacter sphaeroides are not functionally interchangeable

Abstract

SummaryThe s s s s 54 factor is highly conserved in a large number of bacterial species. From the complete genome sequence of Rhodobacter sphaeroides , it was possible to identify four different sequences encoding potentially functional s s s s 54 factors. In this work, we provide evidence that one of these copies ( rpoN 2) is specifically required to express the flagellar genes in this bacterium. A mutant strain carrying a lesion in the rpoN2 gene was unable to swim even though the RpoN1 and RpoN3 proteins were present in the cytoplasm. The possibility that the different copies of the s s s s 54 factor might be specific for the transcription of a particular subset of s s s s 54 promoters was reinforced by the fact that a mutant strain carrying a lesion in rpoN1 showed a severe growth defect in nitrogenfree culture medium, even though the rpoN2 and rpoN4 genes were actively transcribed from a plasmid or from the chromosome. Different mechanisms that might be responsible for this specificity are discussed.

A Complete Set of Flagellar Genes Acquired by Horizontal Transfer Coexists with the Endogenous Flagellar System inRhodobacter sphaeroides

“…Anaerobic growth conditions seem to be the main requirement for induction of swimming of the SP18 fla2ϩ strain since no swimming cells were observed in aerobic cultures of this strain. However, it should be emphasized that anaerobic conditions per se are not sufficient to induce expression of fla2 cluster genes in wild-type R. sphaeroides, since in accordance with previous reports, no swimming cells were detected when different fla1 mutant strains were grown under these conditions (41). In line with these results, the FlgE2 protein was not present in cell extracts obtained from the wild-type WS8 strain grown under anaerobic conditions (Fig.…”

The Flagellar Protein FliL Is Essential for Swimming in Rhodobacter sphaeroides

“…The fragment carrying the ⌬fliL3::aadA allele was subcloned into pJQ200mp18 and introduced into WS8 by conjugation with the S17-1 strain (50). The double recombination events were selected as described previously (49).…”

Flagellar Biogenesis of Xanthomonas campestris Requires the Alternative Sigma Factors RpoN2 and FliA and Is Temporally Regulated by FlhA, FlhB, and FlgM

“…Most bacteria have only one rpoN gene, yet exceptions have been found in some nitrogen fixation bacteria and plant pathogens. The functions of the rpoN homologs in the same genome could be redundant or irrelevant (34,46,59). An allelic-exchange mutation in rpoN2 totally abolishes flagellar biogenesis and motility.…”