TlpC, a novel chemotaxis protein in Rhodobacter sphaeroides, localizes to a discrete region in the cytoplasm

TIpC is encoded in the second chemotaxis operon of Rhodobacter sphaeroides. This protein shows some homology to membrane-spanning chemoreceptors of many bacterial species but, unlike these, is essential for R. sphaeroides chemotaxis to all compounds tested. Genomic replacement of t/pC with a C-terminal gfp fusion demonstrated that TIpC localized to a discrete cluster within the cytoplasm. Immunogold electron microscopy also showed that TIpC localized to a cytoplasmic electron-dense region. Correct TIpC-GFP localization depended on the downstream signalling proteins, CheW(3), CheW(4) and CheA(2), and was tightly linked to cell division. Newly divided cells contained a single cluster but, as the cell cycle progressed, a second cluster appeared close to the initial cluster. As elongation continued, these clusters moved apart so that, on septation, each daughter cell contained a single TIpC cluster. The data presented suggest that TIpC is either a cytoplasmic chemoreceptor responding to or integrating global signals of metabolic state or a novel and essential component of the chemotaxis signalling pathway. These data also suggest that clustering is essential for signalling and that a mechanism may exist for targeting and localizing proteins within the bacterial cytoplasm.