The Yop proteins of Yersinia are important virulence determinants. The Yop1 protein sequences of Yersinia pestis, Yersinia pseudotuberculosis, and two Yersinia enterocolitica serotypes, 0:3 and 0:8, deduced from the nucleotide sequences of the corresponding yopA genes, were compared. Most differences were found in the hydrophilic domains of the proteins, whereas the hydrophobic domains were conserved. The amino acid sequences revealed a signal sequence 25 amino acids long. No cysteine residues were present, even though Yop1 forms a polymeric structure. The transcription startpoint of yopA was determined by primer extension. The coding region and transcription startpoint were separated by a leader sequence 270 nucleotides long. The yopA promoter sequence of Y.pestis is identical to the corresponding sequence of Y. pseudotuberculosis and transcription studies revealed that this promoter is active in Y.pestis. Thus, the inability of Y.pestis to express the Yop1 protein is due to a single base pair deletion in the coding region of the yopA gene of Y.pestis.