Evidence for insertion sequence-mediated spread of the thermostable direct hemolysin gene among Vibrio species.

The tdh gene of Vibrio parahaemolyticus which encodes the thermostable direct hemolysin has been found in some strains of other Vibrio species. Analysis of seven tdh genes cloned from V. parahaemolyticus, Vibrio mimicus, and non-O1 Vibrio cholerae revealed that all tdh genes were flanked by insertion sequence-like elements (collectively named ISVs) or related sequences derived from genetic rearrangement of ISVs. The ISVs possessed 18-bp terminal inverted repeats highly homologous to those of IS903 (2- to 4-bp mismatch) and were 881 to 1,058 bp long with less than 33.6% sequence divergence. These features and nucleotide sequence similarities among ISVs and IS903 (overall homologies between ISVs and IS903, ca. 50%) strongly suggest that they were derived from a common ancestral sequence. A family of ISVs were widely distributed in Vibrio species, often regardless of the possession of the tdh genes, and one to several copies of the ISVs per organism were detected. A strain of V. mimicus possessed two copies of the ISVs flanking the tdh gene and three copies unrelated to the tdh gene. However, the transposition activity of the ISVs could not be demonstrated, probably because they had suffered from base changes and insertions and deletions within the transposase gene. The possible mode of ISV-mediated spread of the tdh gene is discussed from an evolutionary standpoint.