A gene, dmcA, expressing a methyl-accepting chemotaxis protein (MCP) from the oral spirochete Treponema denticola has been characterized. The gene was initially identified as an open reading frame immediately upstream from the previously characterized prtB protease gene of strain ATCC 35405. Nucleotide sequencing of the dmcA gene revealed a potential 57-kDa protein product with extensive homology with the signaling regions of MCPs from a variety of bacteria. The protein expressed in Escherichia coli cross-reacted with anti-Trg (E. coli MCP) serum, confirming its homology with MCPs. Northern blot and primer extension analyses identified the transcription start site of the monocistronic dmcA mRNA. By utilizing a T. denticola gene inactivation system recently developed in this laboratory, a mutant defective in the dmcA gene, HL0501, was constructed. The mutant was demonstrated to be defective in chemotaxis toward nutrients. In addition, the methylation profiles of cellular proteins indicated altered MCPs in the mutant relative to those of the parental strain. These results indicate that we have identified an MCP gene in the oral spirochete which plays a significant role in the chemotactic response of the organism.