RsmA and AmrZ orchestrate the assembly of all three type VI secretion systems in Pseudomonas aeruginosa

Significance

Bacteria evolved molecular weapons to help them thrive in polymicrobial environments. The type VI secretion system (T6SS) is a gun loaded with a great diversity of bacterial toxins. On contact with neighboring cells, toxins are fired, and in the absence of immunity, the prey is killed, allowing the attacker to prevail. Each bacterium can be equipped with several distinct T6SSs, and it is unclear whether they are simultaneously active or whether each has a specific role in a particular environment. Here we showed that production of the three Pseudomonas aeruginosa T6SSs is orchestrated by global regulators. We suggest it may be possible for simultaneous assembly of multiple T6SSs within a single cell, priming it to fight a wide variety of organisms.

Abstract

The type VI secretion system (T6SS) is a weapon of bacterial warfare and host cell subversion. The Gram-negative pathogen Pseudomonas aeruginosa has three T6SSs involved in colonization, competition, and full virulence. H1-T6SS is a molecular gun firing seven toxins, Tse1–Tse7, challenging survival of other bacteria and helping P. aeruginosa to prevail in specific niches. The H1-T6SS characterization was facilitated through studying a P. aeruginosa strain lacking the RetS sensor, which has a fully active H1-T6SS, in contrast to the parent. However, study of H2-T6SS and H3-T6SS has been neglected because of a poor understanding of the associated regulatory network. Here we performed a screen to identify H2-T6SS and H3-T6SS regulatory elements and found that the posttranscriptional regulator RsmA imposes a concerted repression on all three T6SS clusters. A higher level of complexity could be observed as we identified a transcriptional regulator, AmrZ, which acts as a negative regulator of H2-T6SS. Overall, although the level of T6SS transcripts is fine-tuned by AmrZ, all T6SS mRNAs are silenced by RsmA. We expanded this concept of global control by RsmA to VgrG spike and T6SS toxin transcripts whose genes are scattered on the chromosome. These observations triggered the characterization of a suite of H2-T6SS toxins and their implication in direct bacterial competition. Our study thus unveils a central mechanism that modulates the deployment of all T6SS weapons that may be simultaneously produced within a single cell.