Amino acid sequence comparison suggests that numerous proteins are com
mon to the signal transduction pathways controlling chemotaxis in Baci
llus subtilis and Escherichia coli. However, previous work has indicat
ed several differences between the two systems. We have undertaken a c
omparative study of the roles of the CheY protein in chemotaxis by B.
subtilis and E. coli. Although CheY from the two species share only 36
% amino acid sequence identity, purified B. subtilis CheY was phosphor
ylated in vitro by E. coli CheA, and dephosphorylation of CheY-P was e
nhanced by E. coli CheZ. Alteration of the putative site of phosphoryl
ation in B. subtilis CheY, Asp54, eliminated chemotaxis in vivo, furth
er confirming that phosphorylation is important for B. subtilis chemot
axis. Loss of CheY function resulted in tumbling behavior in B. subtil
is. Introduction of positively charged residues in place of Asp10 of B
. subtilis CheY abolished function, whereas the corresponding changes
in E. coli CheY apparently result in constitutive activation. The B. s
ubtilis CheY Asp10 mutant proteins also failed to cause tumbling in E.
coli, consistent with a different interaction between CheY and the fl
agellar switch in the two species. Finally, B. subtilis adapted more r
apidly to positive stimuli than negative stimuli, whereas the opposite
is true of E. coli. We conclude that B. subtilis regulates its respon
se to positive chemotactic stimuli by enhancing phosphorylation of che
motaxis proteins, whereas E. coli reduces phosphorylation in the same
circumstance.