The four different σ 54 factors of Rhodobacter sphaeroides are not functionally interchangeable

Abstract

The σ 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 σ 54 factors. In this work, we provide evidence that one of these copies (rpoN2) 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 σ 54 factor might be specific for the transcription of a particular subset of σ 54 promoters was reinforced by the fact that a mutant strain carrying a lesion in rpoN1 showed a severe growth defect in nitrogen-free 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.