Two Functionally Deviating Type 6 Secretion Systems Occur in the Nitrogen-Fixing Endophyte Azoarcus olearius BH72

Abstract

Type VI protein secretion systems (T6SSs) have been identified in many plant-associated bacteria. However, despite the fact that effector proteins may modulate host responses or interbacterial competition, only a few have been functionally dissected in detail. We dissected the T6SS in Azoarcus olearius strain BH72, a nitrogen-fixing model endophyte of grasses. The genome harbors two gene clusters encoding putative T6SSs, tss-1 and tss-2, of which only T6SS-2 shared genetic organization and functional homology with the H1-T6SS of Pseudomonas aeruginosa. While tss-2 genes were constitutively expressed, tss-1 genes were strongly up-regulated under conditions of nitrogen fixation. A comparative analysis of the wild type and mutants lacking either functional tss-1 or tss-2 allowed to differentiate the functions of both secretion systems. Abundance of Hcp in the culture supernatant as an indication for T6SS activity revealed that only T6SS-2 was active, either under aerobic or nitrogen-fixing conditions. Our data show that T6SS-2 but not T6SS-1 is post-translationally regulated by phosphorylation mediated by TagE/TagG (PpkA/PppA), and by the phosphorylation-independent inhibitory protein TagF, similar to published work in Pseudomonas. Therefore, T6SS-1 appears to be post-translationally regulated by yet unknown mechanisms. Thus, both T6SS systems appear to perform different functions in Azoarcus, one of them specifically adapted to the nitrogen-fixing lifestyle.

Complete Genomes of the Anaerobic Degradation Specialists <b><i>Aromatoleum petrolei</i></b> ToN1^T and <b><i>Aromatoleum bremense</i></b> PbN1^T

“…Activity of T6SS can be posttranslationally modulated via a threonine phosphorylation (TPP) dependent (involving TagEGH) [Mougous et al, 2007] and an independent (involving TagF) [Silverman et al, 2011] pathway. The recent study by Jiang et al [2019] revealed the presence of two gene clusters for T6SS in Az. olearius BH72, termed tss-1 and tss-2 ( Fig.…”

“…petrolei ToN1 T and Ar. bremense PbN1 T was assessed for clues on endophytic lifestyle capacities by searching for the presence of genes required or suspected for Azoarcus-type microbe-plant interaction, i.e., nitrogenase [Egener et al, 1999], type VI protein secretion systems (T6SSs) [Jiang et al, 2019], biosynthesis of indoleacetate [Fernández et al, 2014], pili, flagella, and the TonB-system [Reinhold-Hurek et al, 2006;Böhm et al, 2007;Buschart et al, 2012]. (i) Nitrogenase Diazotrophic microorganisms fix nitrogen mostly by means of molybdenum nitrogenases composed of two protein complexes [for overview refer to Ludden, 2005, 2008].…”

“…The T6SS has broadly been recognized to translocate antibacterial proteins [Hachani et al, 2016], but was recently also implicated in the endophytic lifestyle of Az. olearius BH72 [Shidore et al, 2012;Jiang et al, 2019]. T6SSs are composed of 13 conserved, essential components and structurally organized in two complexes (Fig.…”

“…bremense PbN1 T . a Scheme of the subcellular architecture of T6SSs (modified from Costa et al [2015] and Jiang et al [2019]). b Organization of the two paralogous tss gene clusters in Az.…”

The Role of Secretion Systems, Effectors, and Secondary Metabolites of Beneficial Rhizobacteria in Interactions With Plants and Microbes

“…However, the researchers could not show involvement in inter-bacterial competition in vitro. The nitrogen fixing bacteria Azoarcus olearius BH72 encodes two T6SS operons, one of which is strongly up-regulated when nitrogen is absent (Jiang et al, 2019). Kosakonia strains are endophytic nitrogen fixers involved in plant growth promotion in rice (Bertani et al, 2016).…”