Sensory Transduction in Escherichia coli: Regulation of the Demethylation Rate by the CheA Protein

The reversible methylation of three membrane proteins plays an essential role in bacterial chemotaxis. Chemotactic stimuli bring about changes in the levels of methylation of these proteins, at least in part, by regulation of the demethylation reaction. Addition of attractants causes an increase in the methylation level and a transient, but essentially complete, inhibition in the rate of the demethylation reaction, while addition of repellents results in a decrease in level and a transient increase (of at least 25- to 30-fold) in rate. We have now found that the increase, but not the decrease, in rate requires the presence of the cheA gene product, a protein that is distinct from the demethylase. The demethylation reaction is therefore regulated by two distinct mechanisms--one, which involves the CheA protein, that mediates the increase in rate and a second, which does not involve the CheA protein, that mediates the decrease in rate. Several pieces of evidence already in the literature imply that the CheA protein functions downstream of the methylation system at the flagellar end of the chemotactic machinery. These data, in conjunction with the newer results, suggest that the CheA protein helps to regulate the demethylation reaction through a feedback mechanism.