Abstract
BackgroundThe Type VI secretion apparatus is assembled by a conserved set of proteins encoded within a distinct locus. The putative effector proteins Hcp and VgrG are also encoded within these loci. We have identified numerous distinct Type VI secretion system (T6SS) loci in the genomes of several ecologically diverse Pantoea and Erwinia species and detected the presence of putative effector islands associated with the hcp and vgrG genes.ResultsBetween two and four T6SS loci occur among the Pantoea and Erwinia species. While two of the loci (T6SS-1 and T6SS-2) are well conserved among the various strains, the third (T6SS-3) locus is not universally distributed. Additional orthologous loci are present in Pantoea sp. aB-valens and Erwinia billingiae Eb661. Comparative analysis of the T6SS-1 and T6SS-3 loci showed non-conserved islands associated with the vgrG and hcp, and vgrG genes, respectively. These regions had a G+C content far lower than the conserved portions of the loci. Many of the proteins encoded within the hcp and vgrG islands carry conserved domains, which suggests they may serve as effector proteins for the T6SS. A number of the proteins also show homology to the C-terminal extensions of evolved VgrG proteins.ConclusionsExtensive diversity was observed in the number and content of the T6SS loci among the Pantoea and Erwinia species. Genomic islands could be observed within some of T6SS loci, which are associated with the hcp and vgrG proteins and carry putative effector domain proteins. We propose new hypotheses concerning a role for these islands in the acquisition of T6SS effectors and the development of novel evolved VgrG and Hcp proteins.
Highlights
The Type VI secretion apparatus is assembled by a conserved set of proteins encoded within a distinct locus
Several orthologous T6SS loci occur in Pantoea and Erwinia species By means of baiting with the conserved proteins from several characterized T6SS in other bacteria, between two and four T6SS loci were identified in the complete or draft genome sequences of five Pantoea and four Erwinia strains (Figure 1)
Additional T6SS loci were identified in the genome sequences of E. billingiae Eb661 and Pantoea sp. aB-valens, respectively
Summary
The Type VI secretion apparatus is assembled by a conserved set of proteins encoded within a distinct locus. The Type VI secretion system (T6SS) was first identified in the human pathogens Pseudomonas aeruginosa and Vibrio cholerae and was shown in the latter to function as an injectisome for the delivery of pathogenicity effector proteins into the host cell [1,2]. The genomes of P. aeruginosa, Y. pestis and Burkholderia pseudomallei encode three, four and six T6SS loci, respectively [3] These seemingly non-paralogous loci, together with the divergent functions of the T6SS suggests that this secretory system could influence a variety of interactions within a single species, with various hosts, and/or with other bacteria occupying the same niche [3]
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