Attractants bind to the receptor-transducer proteins ( Tar, tsr, Trg, Tap) in the membrane. The receptors also mediate the response to certain repellent. This activates a chew-dependent change in the rate of autophosphorylation of CheA and the proteins that are phosphorylated by CheA (CheB and CheY). Attractants reduce the rate of autophosphorylation, and repellents increase the rate of autophosphorylation. The phosphoryl group is transferred from CheA-P to CheY and CheB. CheY-P interacts with the switch proteins (FilM, FliN, FliG) to cause clockwise rotation of the motor and tumbling. Thus repellents increase tumbling because they increase the level of CheY-P, and attractants reduce tumbling because they reduce the level of CheY-P. CheB-P is a methylesterase whose activity results in demethylation of the receptor-transducer proteins. As CheB-P goes up, methylation goes down. Thus repellents reduce the level of methylation because they increase the level of CheB-P and attractants increase methylation because they reduce the level of CheB-P. CheB-P autodephosphorylates. CheR is a methyltransferase that methylates the receptor-transducer proteins. The more highly methylated receptor-transducer protein does not transmit the chemoattracant signal to CheA. Hence adapation to a chemoattracant is due to increased methylation of the receptor-transducer proteins. Adapation to a repellent is due to undermethylation of the receptor-transducer protein. The relative activities of CheA-P determine the level of CheY-P, hence the frequency of tumbling (2).