Similarity of the tdh gene-bearing plasmids of Vibrio cholerae non-O1 and Vibrio parahaemolyticus.

The gene encoding a hemolysin similar to the thermostable direct hemolysin (TDH) of Vibrio parahaemolyticus was previously cloned from a plasmid of Vibrio cholerae non-O1. The gene (designated as NAG-tdh) was subcloned and its nucleotide sequence was determined and compared with reported sequences of the four tdh gene copies encoding TDH, of which three were cloned from the chromosome and one was cloned from a plasmid of V. parahaemolyticus. In the coding region, the NAG-tdh gene had 100% homology with the plasmid-borne tdh gene (tdh4) whereas the NAG-tdh gene was 96.7-98.6% homologous to the three chromosomal tdh genes. The sequences of the NAG-tdh and tdh4 genes were nearly identical in the further upstream and downstream regions. The entire plasmids carrying the two tdh genes were found to be highly homologous when compared by restriction endonuclease and Southern blot analyses. The results suggest that the tdh gene has been transferred between V. cholerae non-O1 and V. parahaemolyticus by a plasmid, directly or indirectly, and that the nucleotide sequences of the tdh gene-bearing plasmids have undergone minor base changes in the respective genetic backgrounds.