The role of methylation in chemotaxis. An explanation of outstanding anomalies

The role of methylation in chemotaxis is understood generally, but several anomalies exist which bring into question the timing of methylation relative to sensing. A double mutant bacterium, deficient in both methyltransferase and methylesterase (Tr-Es-) is capable of chemotaxis even though the respective single mutants (Tr- and Es-) are not. This Tr-Es- mutant will accumulate in capillaries containing aspartic acid but not in capillaries containing serine despite the fact that both the aspartate and serine receptors are part of the methylation-dependent pathway. To understand these anomalies, a combination of theoretical analyses and experimental studies was performed. A mathematical analysis of the gradients of aspartate and serine in the capillary assay shows that outside the capillary the gradients are shallow, but just inside the mouth of the capillary they are very steep. Also, when the number of bacteria accumulated in the capillary is at a maximum, the range of attractant concentrations in the steep gradient just inside the mouth of the capillary is optimal for response and partial adaptation by the Tr-Es- mutant. We postulate that random motion brings the Tr-Es- mutant into the capillary, where it is able to move up the steep gradient. The difference in timing of the responses to serine and aspartate explains why the Tr-Es- mutant accumulates in aspartate- but not in serine-containing capillaries. A simple diffusion-capture model incorporating these concepts can account for experimental values of the number of Tr-Es- bacteria accumulating in the capillary. These studies provide a rational explanation for all of the apparent anomalies and lead to the conclusion that methylation/demethylation plays a crucial role in sensing as well as setting the zero point of the receptor.