Localization and environmental regulation of MCP-like proteins in Rhodobacter sphaeroides

Chemotaxis to many compounds by Rhodobacter sphaeroides requires transport and at least partial metabolism of the chemoeffector, Previous investigatio ns using phototrophically grown cells have failed to find any homologues of the MCP chemoreceptors identified in Escherichia coli, However, using an a ntibody raised against the highly conserved domain of E. coli Tsr, MCP-like proteins were identified in R. sphaeroides WS8N, Analysis using Western bl otting and immunogold electron microscopy showed that expression of these M CP-like proteins is environmentally regulated and that receptors are target ed to two different cellular locations: the poles of the cells and the cyto plasm. In aerobically grown cells, these proteins were shown by immunoelect ron microscopy to localize predominantly to the cell poles and to an electr on-dense body in the cytoplasm, Western blot analysis indicated a 17-fold r eduction in protein concentration when cells were grown in the light. The n umber of immunogold particles was also dramatically reduced in anaerobicall y light-grown cells and their cellular distribution was altered. Fewer rece ptors localized to the cell poles and more particles randomly distributed w ithin the cell, but the cytoplasmic cluster remained. These trends were mor e pronounced in cells grown anaerobically under dim light than in those gro wn anaerobically under bright light, suggesting that expression is controll ed by redox state and either light intensity or the extent of photosyntheti c membrane synthesis. Recent work on E. coli chemosensing suggests that oli gomerization of receptors and chemosensory proteins is important for sensor y signalling. The data presented here suggest that this oligomerization can occur with cytoplasmic receptors and also provides an explanation for the multiple copies of chemosensory proteins in R. sphaeroides.