Identification of Escherichia coli ClpAP in regulating susceptibility to type VI secretion system-mediated attack by Agrobacterium tumefaciens

Type VI secretion system (T6SS) is an effector delivery system used by gram-negative bacteria to kill other bacteria or eukaryotic host to gain fitness. In Agrobacterium tumefaciens, T6SS has been shown to kill other bacteria such as Escherichia coli. Interestingly, the A. tumefaciens T6SS killing efficiency differs when using different E. coli strains as recipient cells. Thus, we hypothesize that a successful T6SS killing not only relies on attacker T6SS activity but also depends on recipient factors. A high-throughput interbacterial competition assay was employed to test the hypothesis by screening for mutants with reduced killing outcomes caused by A. tumefaciens strain C58. From the 3909 E. coli Keio mutants screened, 16 candidate mutants were filtered out. One strain, {Delta}clpP::Kan, showed ten times more resistant to T6SS-mediating killing but restored its susceptibility when complemented with clpP in trans. ClpP is a universal and highly conserved protease that exists in both prokaryotes and eukaryotic organelles. In E. coli, ClpP uses either ClpA or ClpX as an adaptor for substrate specificity. Therefore, the susceptibility of the {Delta}clpA::Kan and {Delta}clpX::Kan was also tested. The T6SS attack susceptibility of {Delta}clpA::Kan is at the same level as {Delta}clpP::Kan, while {Delta}clpX::Kan showed no difference as compared to that of wild-type E. coli BW25113. The data also suggest that ClpA-ClpP interaction, rather than its protease activity, is responsible for enhancing susceptibility to T6SS killing. This study highlights the importance of recipient factors in determining the outcome of T6SS killing.