Abstract
Type VI secretion systems (T6SS) enable bacteria to engage neighboring cells in contact-dependent competition. In Vibrio cholerae, three chromosomal clusters each encode a pair of effector and immunity genes downstream of those encoding the T6SS structural machinery for effector delivery. Different combinations of effector-immunity proteins lead to competition between strains of V. cholerae, which are thought to be protected only from the toxicity of their own effectors. Screening of all publically available V. cholerae genomes showed that numerous strains possess long arrays of orphan immunity genes encoded in the 3′ region of their T6SS clusters. Phylogenetic analysis reveals that these genes are highly similar to those found in the effector-immunity pairs of other strains, indicating acquisition by horizontal gene transfer. Extensive genomic comparisons also suggest that successive addition of effector-immunity gene pairs replaces ancestral effectors, yet retains the cognate immunity genes. The retention of old immunity genes perhaps provides protection against nearby kin bacteria in which the old effector was not replaced. This mechanism, combined with frequent homologous recombination, is likely responsible for the high diversity of T6SS effector-immunity gene profiles observed for V. cholerae and closely related species.
Highlights
The family Vibrionaceae consists of over 100 related species of highly motile, heterotrophic bacteria that enzymatically convert inaccessible organic matter found in aquatic environments into carbon sources available to higher trophic levels of the ecosystem they inhabit[1]
Initial screening for T6SS clusters in Vibrio cholerae and the closely related Vibrio metoecus, Vibrio mimicus, Vibrio fluvialis, and Vibrio furnissii was conducted by performing megaBLAST searches against all genomes of these species (V. cholerae: 548, V. metoecus: 10, V. mimicus: 10, V. fluvialis: 8, V. furnissii: 4) available on NCBI
Our observation of mosaic arrays of immunity genes in the T6SS clusters of V. cholerae and closely related species suggests a selective advantage for the presence of multiple immunity genes
Summary
The family Vibrionaceae consists of over 100 related species of highly motile, heterotrophic bacteria that enzymatically convert inaccessible organic matter found in aquatic environments into carbon sources available to higher trophic levels of the ecosystem they inhabit[1]. The resulting “poisoned” spear proves lethal to target cells that do not possess an EI module of the same type[17] Through this system, strains of V. cholerae are able to attack eukaryotes and bacteria belonging to different species, and their perhaps strongest competitors, non-kin strains of the same species[19]. The large number of distinct EI modules may indicate an ongoing evolutionary arms race to succeed in intraand interspecies competition and to overcome eukaryotic host defenses Such intense selective pressure could facilitate rapid mutational divergence of effectors and immunity proteins of different lineages, and horizontal gene transfer, either as a whole or in parts, giving rise to new variants
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