Co-evolution of Hcp and VgrG Proteins of Type Ⅵ Secretion System

Key words: 
• Gram-negative bacterium /
• Xanthomonas oryzae pv. oryzicola /
• T6SS /
• Hcp /
• VgrG

Abstract: The Type Ⅵ secretion system（T6SS） is widely distributed amongst diverse Gram-negative bacterial species.The extracellular components of the T6SS,Hcp and VgrG,form a needle-like injection device that resembles the phage tail.Hcp forms hexameric rings（sheath） and VgrG at the distal tip of the sheath forms a tail spike-like cell-puncturing device.The relationship between Hcp and VgrG was analyzed at evolutional and expressional levels.Results showed that Hcp co-evolved with other T6SS genes while VgrG evolved randomly.One bacterial species may encode more than one T6SS.The number of Hcp is often equal to that of T6SS,but VgrG is not at all the case.A bacterium encoding T6SS may have up to 30 vgrG genes which are not related to the number of T6SS.Based on the evolution and homology analysis,Hcp can be classified into 14 types,while VgrG into 3 classes.The orthologs of Hcp and VgrG have higher conservation than the paralogs.The VgrG is believed to act as a carrier of effectors.Thus,the diversity of VgrG might be the reason that a bacterium encodes different effectors to kill different bacterial species.Co-evolution analysis showed that Hcp and VgrG were closely related during evolution,with predictable high affinity partner between Hcp and vgrG genes.Expression analysis showed that expression of almost all VgrG genes was positively correlated with the Hcp genes in Xanthomonas oryzae pv.oryzicola RS105 which encodes 30 VgrGs,indicating the expression of both VgrG and Hcp can not be used for their relation analysis but for functional relation analysis in RS105.The results support that Hcp and VgrG are functional and evolutional partners in Gram-negative bacteria.