Bacterial chemotaxis:: Rhodobacter sphaeroides and Sinorhizobium meliloti -: variations on a theme?

We are only beginning to understand the mechanisms involved in tactic sensing in the α-subgroup of bacteria. It is clear, however, from recent developments that although the central chemosensory pathways are related to those identified in enteric species, the primary signals and the effect on flagellar behaviour are very different. The expression of chemoreceptors is under environmental control, and the strength of a response depends on the metabolic state of the cell. This is very different from enteric species which always respond to MCP-dependent chemoeffectors, and in which the expression of the receptors is constitutive. Chemotaxis in R. sphaeroides and S. meliloti is therefore more directly linked to the environment in which a cell finds itself. The integration of chemosensory pathways dependent on growth state may be much more suited to the fluctuating environment of these soil and water bacteria. There is still a great deal that needs to be understood about the mechanisms involved in motor control. The presence of at least two CheY homologues and the finding that the swimming speed of these bacteria can vary, and, in the case of S. meliloti, vary with chemosensory stimulation suggests a different control mechanism at the flagellar motor where speed can be altered, or the motor stopped, with a full Δp still present. Why R. sphaeroides should have at least two functional sets of genes encoding homologues of the enteric chemosensory pathway remains to be determined. The major differences in sensory behaviour between the two α-subgroup species so far studied in detail and the differences from the enteric species suggests that many more variations of the chemosensory pathways will be found as more species are studied.