Diversity and distribution of Type VI Secretion System gene clusters in bacterial plasmids

Abstract

Type VI Secretion System (T6SS) is a nanomolecular apparatus that allows the delivery of effector molecules through the cell envelope of a donor bacterium to prokaryotic and/or eukaryotic cells, playing a role in the bacterial competition, virulence, and host interaction. T6SS is patchily distributed in bacterial genomes, suggesting an association with horizontal gene transfer (HGT). In fact, T6SS gene loci are eventually found within genomic islands (GIs), and there are some reports in plasmids and integrative and conjugative elements (ICEs). The impact that T6SS may have on bacteria fitness and the lack of evidence on its spread mechanism led us to question whether plasmids could represent a key mechanism in the spread of T6SS in bacteria. Therefore, we performed an in-silico analysis to reveal the association between T6SS and plasmids. T6SS was mined on 30,660 plasmids from NCBI based on the presence of at least six T6SS core proteins. T6SS was identified in 330 plasmids, all belonging to the same type (T6SSi), mainly in Proteobacteria (328/330), particularly in Rhizobium and Ralstonia. Interestingly, most genomes carrying T6SS-harboring plasmids did not encode T6SS in their chromosomes, and, in general, chromosomal and plasmid T6SSs did not form separate clades.

The type-VI secretion system of the beneficial symbiont Vibrio fischeri

“…Fig. 1b) [27,28]. Approximately 25 % of all sequenced Gram-negative bacteria encode these gene clusters [27], which suggests that T6SSs function in a plethora of ecological networks.…”

“…The 13 core components are labelled with the suffix Tss, which stands for T ype s ix s ecretion () [1], and their genes are typically co-localized within gene clusters (e.g. ) [27, 28]. Approximately 25 % of all sequenced Gram-negative bacteria encode these gene clusters [27], which suggests that T6SSs function in a plethora of ecological networks.…”

T6SS-mediated competitive exclusion amongPantoea agglomeransassociated with plants

“…We find that all eleven strains harbor up to three distinct T6SS-related gene clusters in their chromosomes. In contrast to other plant-associated enterobacterial species [54], no T6SS-related gene clusters were detected on the genus-specific plasmids, neither on the genus-specific large Pantoea plasmid (LPP) nor the second large plasmid that is characteristic for P. agglomerans . Indeed, all eleven strains effectively kill vulnerable E. coli cells lacking the immunity profile to any of the attacking strains.…”

Social Diversification Driven by Mobile Genetic Elements

“…Additionally, toxin-delivery systems are also found on plasmids. For instance, Morgado and Vicente [ 72 ] identified 330 plasmids, primarily in the Proteobacteria, that encode T6SS genes. T6SS genes are also encoded on integrative conjugative elements (ICEs) and genomic islands, suggesting HGT [ 72 , 73 ].…”

“…For instance, Morgado and Vicente [ 72 ] identified 330 plasmids, primarily in the Proteobacteria, that encode T6SS genes. T6SS genes are also encoded on integrative conjugative elements (ICEs) and genomic islands, suggesting HGT [ 72 , 73 ]. Recently, a T6SS operon in Vibrio fischeri was found that functions in KD [ 74 ] and contains a putative lipoprotein that specifies target cells [ 75 ].…”