Bacteria swim in liquid environments by means of a complex rotating structure known as the flagellum. Approximately 40 proteins are required for the assembly and functionality of this structure. Rhodobacter sphaeroides has two flagellar systems. One of these systems has been shown to be functional and is required for the synthesis of the well-characterized single subpolar flagellum, while the other was found only after the genome sequence of this bacterium was completed. In this work we found that the second flagellar system of R. sphaeroides can be expressed and produces a functional flagellum. In many bacteria with two flagellar systems, one is required for swimming, while the other allows movement in denser environments by producing a large number of flagella over the entire cell surface. In contrast, the second flagellar system of R. sphaeroides produces polar flagella that are required for swimming. Expression of the second set of flagellar genes seems to be positively regulated under anaerobic growth conditions. Phylogenic analysis suggests that the flagellar system that was initially characterized was in fact acquired by horizontal transfer from a ␥-proteobacterium, while the second flagellar system contains the native genes. Interestingly, other ␣-proteobacteria closely related to R. sphaeroides have also acquired a set of flagellar genes similar to the set found in R. sphaeroides, suggesting that a common ancestor received this gene cluster.The bacterial flagellum is a complex protein structure which consists of a long helical filament that is connected through a flexible linker known as the hook to an H ϩ -or Na ϩ -driven rotary motor (29). Rotation of the filament produces thrust that allows the cell to swim in liquid or semisolid medium (6). Bacteria perform taxis by controlling the frequency of reorientation, which is commonly regulated by a two-component signal transduction system that senses an environmental stimulus (2, 3, 7). Several reorientation mechanisms have been described for different bacteria (35). In Escherichia coli and Salmonella enterica serovar Typhimurium, in which the flagellar system has been studied more extensively, more than 40 proteins are required for flagellar synthesis and functioning (29). Expression of the flagellar genes is regulated in a hierarchical pattern which results from coordination of flagellar gene expression at the transcriptional or posttranscriptional level with one or more structural checkpoints in flagellum biogenesis (31). This tight regulation has probably evolved to avoid unnecessary synthesis of the large number of flagellar protein subunits required for this structure. The high energetic cost required for the synthesis and functioning of the flagellum is compensated for by the selective advantage conferred by motility. Accordingly, motility seems to be important in several processes, such as colonization, pathogenesis, dispersion, and competition for resources (37). When the growth medium is too dense to allow swimming, many bacteria differentiate in...

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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.

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