CONICET | Buscador de Institutos y Recursos Humanos

Chemotaxis is an adaptive behavior that allows bacteria to find optimal conditions for survival. This is achieved through a signal transduction pathway that detects chemical gradients and transmits the information to the flagellar motors in order to control navigation in preferred directions. The pathway is conserved among Bacteria and Archaea and relies in the detection of stimuli by chemoreceptors that typically possess an extracellular ligand binding domain and a highly conserved cytoplasmic signaling domain. They form large polar complexes based in interactions between each other, the coupling protein CheW and the histidine kinase CheA, whose activity is controlled in response to ligand binding. Bacterial genomes code for variable numbers of chemoreceptors, ranging from very few to up to fifty. In most cases, very little is known about individual receptors, which are expected to sense specific ligands through their highly variable extracellular domain. Given the conservation of the signaling domain we hypothesized that it might be possible to use E. coli as a reporter strain in order to study their specificity. The aim of this work was to find out whether chemoreceptors from Rhodobacter sphaeroides are able to form complexes with E. coli chemotaxis proteins and control the activity of CheA in response to ligands. Rhodobacter sphaeroides is a a-subgroup, purple nonsulfur, photosynthetic bacterium. It codes for thirteen chemoreceptors of unknown specificity. We chose two receptors from R. sphaeroides, McpH and McpB, and expressed them in an E. coli strain lacking native receptors. Both receptors were able to form signaling complexes and drive polar localization of a CheA-associated phosphatase fused to YFP. Moreover, these complexes were functional, as they were able to generate clockwise rotation of flagella, which is dependent on CheA kinase activity. Using the in vivo flagellar rotation assay, the cells were challenged with different ligands. Complexes containing McpB or McpH as the only receptor did not show any response to aminoacids or sugars. However, they did respond to the addition of organic acids. Cells expressing McpB inhibited the kinase in response to all organic acids tested; including lactate, pyruvate, acetate, citrate and succinate, indicating that this receptor mediates attractant responses to these stimuli. In contrast, only lactate, pyruvate and acetate mediated responses in cells expressing McpH; and these responses were of repellent type. Moreover, in cells co-expressing a serine receptor from E. coli and McpB, responses to the specific ligands showed collaborative signaling.