Identification of the tip-encoded receptor in bacterial sensing

A chemotaxis gene encoding a protein with receptorlike properties has been identified in Salmonella typhimurium and termed tip for taxis-involved protein. Based on the stringency of DNA hybridization, the tip gene has about 75% homology with a region of the tar gene encoding the cytoplasmic domain of the aspartate receptor. Introduction of the tip gene into a smooth-swimming Escherichia coli receptor mutant (tar tsr tap) restored both chemotaxis ability on soft-agar-tryptone plates and a wild-type swimming phenotype. We have shown, by overexpressing the CheY protein, that shifting of the mutant swimming bias in the absence of receptors is insufficient to restore chemotaxis ability. This suggests that in addition to resetting the swimming bias, the tip gene product functions as a receptor. By functional criteria, we found that Tip is not a duplicate aspartate (Tar) or serine (Tsr) receptor gene. Based on behavioral properties, the S. typhimurium Tip receptor provides functional features similar to those of the E. coli Tap receptor.