Cellular Stoichiometry of Methyl-Accepting Chemotaxis Proteins in Sinorhizobium meliloti

The chemosensory system inSinorhizobium meliloti has several important deviations from the widely studied enterobacterial paradigm. To better understand the differences between the two systems and how they are optimally tuned, we determined the cellular stoichiometry of the methyl-accepting chemotaxis proteins (MCPs) and the histidine kinase CheA inS. meliloti . Quantitative immunoblotting was used to determine the total amount of MCPs and CheA per cell inS. meliloti . The MCPs are present in the cell in high abundance (McpV), low abundance (IcpA, McpU, McpX, and McpW), and very low abundance (McpY and McpZ), whereas McpT was below the detection limit. The approximate cellular ratio of these three receptor groups is 300:30:1. The chemoreceptor-to-CheA ratio is 23.5:1, highly similar to that seen inBacillus subtilis (23:1) and about 10 times higher than that inEscherichia coli (3.4:1). Different fromE. coli , the high-abundance receptors inS. meliloti are lacking the carboxy-terminal NWETF pentapeptide that binds the CheR methyltransferase and CheB methylesterase. Using transcriptionallacZ fusions, we showed that chemoreceptors are positively controlled by the master regulators of motility, VisNR and Rem. In addition, FlbT, a class IIA transcriptional regulator of flagellins, also positively regulates the expression of most chemoreceptors except for McpT and McpY, identifying chemoreceptors as class III genes. Taken together, these results demonstrate that the chemosensory complex and the adaptation system inS. meliloti deviates significantly from the established enterobacterial paradigm but shares some similarities withB. subtilis .IMPORTANCE The symbiotic soil bacteriumSinorhizobium meliloti is of great agricultural importance because of its nitrogen-fixing properties, which enhances growth of its plant symbiont, alfalfa. Chemotaxis provides a competitive advantage for bacteria to sense their environment and interact with their eukaryotic hosts. For a better understanding of the role of chemotaxis in these processes, detailed knowledge on the regulation and composition of the chemosensory machinery is essential. Here, we show that chemoreceptor gene expression inS. meliloti is controlled through the main transcriptional regulators of motility. Chemoreceptor abundance is much lower inS. meliloti than inEscherichia coli andBacillus subtilis . Moreover, the chemoreceptor-to-kinase CheA ratio is different from that ofE. coli but similar to that ofB. subtilis .