Differential CheR affinity for chemoreceptor C-terminal pentapeptides biases chemotactic responses

Abstract

Chemotaxis pathways are sophisticated signal transduction systems in bacteria. Their capacity to respond to gradients of signals relies on adaptation mediated by the coordinated action of CheR methyltransferases and CheB methylesterases that control the methylation state of the signaling domain. Many chemoreceptors contain a C-terminal pentapeptide at the end of a flexible linker. In Escherichia coli, this pentapeptide forms a high-affinity binding site for CheR and phosphorylated CheB, and its removal interferes with adaptation. The analysis of all available chemoreceptor sequences showed that pentapeptide sequences vary greatly, and bacteria often possess multiple chemoreceptors that differ in their pentapeptide sequences. Using the phytopathogen Pectobacterium atrosepticum, we assessed whether this sequence variation alters CheR affinity and affects chemotaxis. The model strain SCRI1043 has 36 chemoreceptors, of which 19 possess a C-terminal pentapeptide. Using isothermal titration calorimetry, we show that the affinity of CheR for the different pentapeptides varies up to 12-fold (KDof 86 nM to 1.04 μM). The pentapeptides with the highest and lowest affinities differed only in a single amino acid. Deletion of the cheR gene abolishes chemotaxis. Using quantitative immunoblot analyses, we demonstrated that the cellular CheR concentration varies during growth and is in the range of the pentapeptide affinities. PacC is the sole chemoreceptor for L-Asp in SCRI1043, and the replacement of its pentapeptide with those having the highest and lowest affinities significantly interfered with L-Asp chemotaxis. Variable C-terminal pentapeptide sequences thus provide a mechanism to bias the responses mediated by chemoreceptors.