The Escherichia coli AtoS-AtoC two-component signal transduction system regulates the expression of the atoDAEB operon genes, whose products are required for short-chain fatty acid catabolism. In this study purified his-tagged wild-type and mutant AtoS proteins were used to prove that these proteins are true sensor kinases. The phosphorylated residue was identified as the histidine-398, which was located in a conserved Η-box since AtoS carrying a mutation at this site failed to phosphorylate. This inability to phosphorylate was not due to gross structural alterations of AtoS since the H398L mutant retained its capability to bind ATP. Furthermore, the H398L mutant AtoS was competent to catalyze the trans-phosphorylation of an AtoS G-box (G565A) mutant protein which otherwise failed to autophosphorylate due to its inability to bind ATP. The formation of homodimers between the various AtoS proteins was also shown by cross-linking experiments both in vitro and in vivo.