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Evidence for insertion sequence-mediated spread of the thermostable direct hemolysin gene among Vibrio species
1Department of Microbiology, Faculty of Medicine, Kyoto University, Japan.
Abstract:
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.
Insights
The tdh gene, encoding a toxin, is found in Vibrio species, often near insertion sequence-like elements (ISVs). These ISVs, likely from a common ancestor, may facilitate the spread of the tdh gene across different Vibrio bacteria.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- The thermostable direct hemolysin (tdh) gene, crucial for Vibrio parahaemolyticus virulence, is also present in other Vibrio species.
- Insertion sequence-like elements (ISVs) have been observed flanking the tdh gene in various Vibrio species.
Purpose of the Study:
- To investigate the genetic characteristics and distribution of ISVs associated with the tdh gene in Vibrio species.
- To explore the evolutionary relationship between ISVs and their potential role in the dissemination of the tdh gene.
Main Methods:
- Cloning and sequencing of seven tdh genes from Vibrio parahaemolyticus, Vibrio mimicus, and non-O1 Vibrio cholerae.
- Analysis of ISVs flanking the tdh genes, including their terminal inverted repeats, length, and sequence divergence.
- Comparative sequence analysis of ISVs and IS903 to determine evolutionary origins.
Main Results:
- All analyzed tdh genes were flanked by ISVs or related sequences, suggesting a common origin.
- ISVs exhibited conserved features like 18-bp terminal inverted repeats and significant homology to IS903, indicating a shared ancestry.
- ISVs were widely distributed in Vibrio species, irrespective of tdh gene presence, with multiple copies detected per organism.
Conclusions:
- ISVs likely originated from a common ancestral sequence and are widespread within the Vibrio genus.
- The genetic features of ISVs suggest their involvement in the spread and potential rearrangement of the tdh gene among Vibrio species.
- While ISVs show characteristics of mobile genetic elements, their transposition activity could not be demonstrated, possibly due to mutations in the transposase gene.