Strategies for differential sensory responses mediated through the same transmembrane receptor

Trg mediates chemotaxis of Escherichia coli to galactose and ribose by recognition of respective, sugar-occupied binding proteins. Although both attractants act through one transmembrane receptor, maximal response is approximately 50% greater to ribose. This phenomenon was investigated by mutational analysis of a 20-residue segment of Trg implicated in ligand interaction and signalling. Among 17 defective receptors, responses to the two chemoattractants were reduced equivalently for seven and differentially for 10, in some cases reversing the preference order. Mutational substitutions with equivalent effects occurred throughout the segment, but those with a greater effect on galactose or ribose response were segregated to the amino-terminal two-thirds or the carboxy-terminal one-third, respectively, a segregation corresponding in large part to a functional division based on signalling phenotypes. A model for binding protein-mediated recognition revealed two strategies for differential responses. The wild-type preference for ribose probably reflects a balance of receptor affinities and a limiting supply of binding proteins. Mutants with reversed preference probably have differentially reduced receptor affinities and those with an accentuated ribose preference probably have altered signalling abilities. Two-step recognition of ligand allows functional separation of detection and response.