Dissecting the bacterial type VI secretion system by a genome wide in silico analysis: what can be learned from available microbial genomic resources?

Abstract

BackgroundThe availability of hundreds of bacterial genomes allowed a comparative genomic study of the Type VI Secretion System (T6SS), recently discovered as being involved in pathogenesis. By combining comparative and phylogenetic approaches using more than 500 prokaryotic genomes, we characterized the global T6SS genetic structure in terms of conservation, evolution and genomic organization.ResultsThis genome wide analysis allowed the identification of a set of 13 proteins constituting the T6SS protein core and a set of conserved accessory proteins. 176 T6SS loci (encompassing 92 different bacteria) were identified and their comparison revealed that T6SS-encoded genes have a specific conserved genetic organization. Phylogenetic reconstruction based on the core genes showed that lateral transfer of the T6SS is probably its major way of dissemination among pathogenic and non-pathogenic bacteria. Furthermore, the sequence analysis of the VgrG proteins, proposed to be exported in a T6SS-dependent way, confirmed that some C-terminal regions possess domains showing similarities with adhesins or proteins with enzymatic functions.ConclusionThe core of T6SS is composed of 13 proteins, conserved in both pathogenic and non-pathogenic bacteria. Subclasses of T6SS differ in regulatory and accessory protein content suggesting that T6SS has evolved to adapt to various microenvironments and specialized functions. Based on these results, new functional hypotheses concerning the assembly and function of T6SS proteins are proposed.

Baseplate assembly of phage Mu: Defining the conserved core components of contractile-tailed phages and related bacterial systems

“…In surveying 280 T6SS operons, we found the tssF and tssG genes located immediately adjacent in 99% of cases (Dataset S5) and the gene encoding the BW1 homolog (TssE) within three ORFs upstream of the tssF gene 93% of the time. Strong genetic linkage between these three genes in a smaller group of T6SS operons was previously reported (21). PVC operons also displayed the same conserved BW1-BW2-BW3 gene order, with these genes being adjacent greater than 90% of the time (Dataset S5).…”

TssK Is a Trimeric Cytoplasmic Protein Interacting with Components of Both Phage-like and Membrane Anchoring Complexes of the Type VI Secretion System

“…Among these genes, tssK encodes an uncharacterized putative soluble protein. The tssK gene is organized in tandem with tssL in 74% of the T6SS gene clusters suggesting a functional link or interaction between these two proteins (8). The association of TssK with the membrane complex is also strongly suggested by the observation that this protein has been found associated with the inner membrane proteome in Pseudomonas aeruginosa (33).…”

“…Regarding this membrane complex, comparative bioinformatic analyses reported a conserved genomic organization between the tssJ, tssK, tssL, and tssM genes (8). Among these genes, tssK encodes an uncharacterized putative soluble protein.…”

A multidrug resistance plasmid contains the molecular switch for type VI secretion in Acinetobacter baumannii

“…A. baumannii is often isolated from polymicrobial infections and therefore spends at least a part of its time competing with other bacteria (4). Antagonistic interactions between bacteria manifest in a variety of different ways (5), and the type VI secretion system (T6SS) is a potent weapon used by many Gram-negative bacteria to kill competitors (6)(7)(8). The multicomponent T6SS apparatus facilitates a dynamic contact-dependent injection of toxic effector proteins into prey cells (9,10), and expression of cognate immunity proteins prevents self-inflicted intoxication (9,11).…”