[Erh-Min Lai and Chih-Horng Kuo] A shared belt-and-braces warfare strategy within Agrobacterium strains

Type VI secretion system (T6SS) is one powerful anti-bacterial and anti-host nanoweapon employed by many Gram-negative bacteria for growth advantages or pathogenesis. A T6SS-harboring bacterium can encode one to multiple VgrG proteins for delivery of cognate effector(s) but the prevalence and biological significance of having sequence redundant vgrG genes have not been comprehensively explored. In this study, the groups of Erh-Min Lai and Chih-Horng Kuo  employed phylogenomic analysis to reveal that the majority of Proteobacterial genomes harbor multiple copies of vgrG but strikingly only a few bacterial genomes have redundant vgrG paralogs. Among them, Agrobacterium tumefaciens 1D1609 has the highest vgrG redundancy. Further study revealed that the expansion of vgrG gene clusters in 1D1609 contributed to effector diversity and functional redundancy. The four vgrGs are functionally redundant in T6SS assembly and 1D1609 uses a nuclease as major antibacterial toxin for interbacterial competition. Importantly, the near identical VgrG proteins between two sibling strains (1D1609 and C58) can be exchanged for effector delivery in killing another competing bacterium such as T6SS-lacking Escherichia coli. This led us to propose that Agrobacterium has developed this flexible mode of T6SS effector delivery - using highly similar VgrGs, either produced endogenously or injected from its close relatives. This flexibility could allow them to be an ally to sister cells and an effective weapon against distantly related bacterial competitors. The knowledge gained from this study has advanced our understanding on the secretion mechanism and strategy evolved in agrobacteria or broadly in many bacterial pathogens to compete within microbial community.

This work is mainly contributed by Nia Santos, a TIGP-MBAS Ph.D. student and published in the journal Frontiers in Microbiology, as part of research topic “Bacterial Secretion Systems.

Santos MNM, Cho S-T, Wu C-F, Chang C-J, Kuo C-H and Lai E-M (2019) Redundancy and Specificity of Type VI Secretion vgrG Loci in Antibacterial Activity of Agrobacterium tumefaciens 1D1609 Strain. Front. Microbiol. 10:3004. doi: 10.3389/fmicb.2019.03004