Abstract
The type VI secretion system (T6SS) is used by many bacteria to engage in social behavior and can affect the health of its host plant or animal. Because activities associated with T6SSs are often costly, T6SSs must be tightly regulated. However, our knowledge regarding how T6SS assembly and contraction are regulated remains limited. Using the plant pathogen Agrobacterium tumefaciens, we show that effectors are not just passengers but also impact on T6SS assembly. The A. tumefaciens strain C58 encodes one T6SS and two Tde DNase toxin effectors used as major weapons for interbacterial competition. Here, we demonstrate that loading of Tde effectors onto their cognate carriers, the VgrG spikes, is required for active T6SS secretion. The assembly of the TssBC contractile sheath occurs only in the presence of Tde effectors. The requirement of effector loading for efficient T6SS secretion was also validated in other A. tumefaciens strains. We propose that such a mechanism is used by bacteria as a strategy for efficacious T6SS firing and to ensure that effectors are loaded onto the T6SS prior to completing its assembly.
Highlights
The type VI secretion system (T6SS) is a versatile secretion system that has been implicated in virulence, antagonism, nutrient acquisition, and horizontal gene transfer [1,2,3]
We found that the secretion of Hcp and VgrG proteins is affected by the presence or absence of Tde effector genes in A. tumefaciens
We describe a new mechanism that applies across the A. tumefaciens group of bacteria
Summary
The type VI secretion system (T6SS) is a versatile secretion system that has been implicated in virulence, antagonism, nutrient acquisition, and horizontal gene transfer [1,2,3]. Effectors are loaded onto the T6SS either via non-covalent interactions (cargo effectors) or as carboxy-terminal extensions to either of the three core structural components (Hcp, VgrG, or PAAR) [2,9]. Cargo effectors are considered as accessory components of the secretion apparatus. This is based on the repeated observations that Hcp and/or VgrG, key markers for T6SS secretion, can be detected in the extracellular milieu of mutants lacking effector genes [10,11,12,13,14]
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