Mutagenic studies of the interaction between the aspartate receptor and methyltransferase from Escherichia coli.

The 4 glutamate residues that are the sites of methylation in the aspartate receptor, which mediates bacterial chemotaxis, were systematically mutated to aspartate residues. None of the aspartate residues were methylated. In each case, the presence of the aspartate mutations altered the methylation rates at glutamate residues on the same receptor. Methylation was nearly eliminated at residues two turns of alpha helix, in the N-terminal direction, from a site of mutation, whereas less severe changes occurred at other positions. The methyltransferase apparently must contact a glutamate or glutamine residue two turns of helix in the C-terminal direction from the site of modification in order to methylate that position at a maximal rate. Mutations from glutamate to aspartate at any of the four sites also appear to alter the methylation rate at the other sites through a change in the structure of the receptor. The aspartate mutations did not substantially alter the affinity of the methyltransferase for the receptor.