Aspartate taxis mutants of the Escherichia coli tar chemoreceptor

The Tar protein of Escherichia coli belongs to a family of methyl-accepting inner membrane proteins that mediate chemotactic responses to a variety of compounds. These transmembrane signalers monitor the chemical environment by means of specific ligand-binding sites arrayed on the periplasmic side of the membrane, and in turn control cytoplasmic signals that modulate the flagellar rotational machinery. The periplasmic receptor domain of Tar senses two quite different chemoeffectors, aspartate and maltose. Aspartate is detected through direct binding to Tar molecules, whereas maltose is detected indirectly when complexed with the periplasmic maltose-binding protein. Saturating levels of either aspartate or maltose do not block behavioral responses to the other compound, indicating that the detection sites for these two attractants are not identical. We initiated structure-function studies of these chemoreceptor sites by isolating tar mutants which eliminate aspartate or maltose taxis, while retaining the ability to respond to the other chemoeffector. Mutants with greatly reduced aspartate taxis are described and characterized in this report. When present in single copy in the chromosome, these tar mutations generally eliminated chemotactic responses to aspartate and structurally related compounds, such as glutamate and methionine. Residual responses to these compounds were shifted to higher concentrations, indicating a reduced affinity of the aspartate-binding site in the mutant receptors. Maltose responses in the mutants ranged from 10 to 80% of normal, but had no detectable threshold shifts, indicating that these receptor alterations may have little effect on maltose detection sensitivity. The mutational changes in 17 mutants were determined by DNA sequence analysis. Each mutant exhibited a single amino acid replacement at residue 64, 69, or 73 in the Tar molecule. The wild-type Tar transducer contains arginines at all three of these positions, implying that electrostatic forces may play an important role in aspartate detection.