BACKGROUND: The 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.
RESULTS: Between 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.
CONCLUSIONS: Extensive 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.