Intraspecies Competition in Serratia marcescens Is Mediated by Type VI-Secreted Rhs Effectors and a Conserved Effector-Associated Accessory Protein.
Bottom Line: Multiple antibacterial effectors can be delivered by these systems, with diverse activities against target cells and distinct modes of secretion.Polymorphic toxins containing Rhs repeat domains represent a recently identified and as-yet poorly characterized class of T6SS-dependent effectors.Furthermore, a new family of accessory proteins associated with T6SS effectors has been identified, exemplified by S. marcescens EagR1, which is specifically required for deployment of its associated Rhs effector.
Affiliation: Division of Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee, United Kingdom.Show MeSH
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Mentions: As mentioned above, we had previously observed T6SS-dependent secretion of SMDB11_2278, a PAAR domain-containing Rhs family protein encoded by a gene within the main T6SS gene cluster of S. marcescens Db10 (Fig. 1A). Further examination of the genome of S. marcescens Db10 revealed the presence of a second such Rhs protein, encoded elsewhere in the genome by SMDB11_1610. Reexamination of our proteomic data from the earlier study (22) indicated that SMDB11_1610 also can be secreted in a T6SS-dependent manner: although missing our stringent quality criteria, since only one peptide was detected, the presence of this peptide in the secreted fraction was strictly T6SS dependent (data not shown). Thus, we named SMDB11_2278 Rhs1 and SMDB11_1610 Rhs2. Examination of the protein sequences of Rhs1 and Rhs2 confirmed the domain organization expected for a T6SS-associated Rhs protein (Fig. 1B). PAAR motifs are present in the N-terminal domain of each protein, followed by a central region containing multiple Rhs repeats and including an Rhs core domain which terminates in the so-called hyperconserved domain (18, 19). Finally, each possesses a distinct C-terminal domain (CT), unrelated between the two proteins, which is predicted to be a toxin or effector domain. The C-terminal domain of Rhs2 (Rhs2-CT) contains a partial HNH endonuclease domain (see Fig. S1 in the supplemental material), suggesting DNase activity, whereas the function of the C-terminal domain of Rhs1 (Rhs1-CT) is not readily apparent from sequence analysis. It is noteworthy that Rhs1 and Rhs2 belong to distinct clades of Rhs-family proteins, clade III and clade II, respectively (18).
Affiliation: Division of Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee, United Kingdom.