Abstract
Mesorhizobium loti MAFF303099 has a functional type III secretion system (T3SS) that is involved in the determination of nodulation competitiveness on Lotus. The M. loti T3SS cluster contains gene y4yS (mlr8765) that codes for a protein of unknown function (Y4yS). A mutation in the y4yS gene favors the M. loti symbiotic competitive ability on Lotus tenuis cv. Esmeralda and affects negatively the secretion of proteins through T3SS. Here we localize Y4yS in the bacterial membrane using a translational reporter peptide fusion. In silico analysis indicated that this protein presents a tetratricopeptide repeat (TPR) domain, a signal peptide and a canonical lipobox LGCC in the N-terminal sequence. These features that are shared with proteins required for the formation of the secretin complex in type IV secretion systems and in the Tad system, together with its localization, suggest that the y4yS-encoded protein is required for the formation of the M. loti T3SS secretin (RhcC2) complex. Remarkably, analysis of RhcC2 in the wild-type and M. loti y4yS mutant strains indicated that the absence of Y4yS affects negatively the accumulation of normal levels of RhcC2 in the membrane.
Highlights
Type III secretion systems (T3SSs) are present in several pathogenic bacteria (Viprey et al, 1998; Cornelis, 2002)
When the needle comes into contact with a host cell, synthesis of a translocation pore composed of different bacterial proteins occurs in the host plasma membrane (Saad et al, 2008)
Taking into account that this assay allows us to infer the effect of T3SS mutation on nodulation phenotype, we compared the competitiveness of the y4yS mutant strain with that of the wild-type strain on L. tenuis cv
Summary
Type III secretion systems (T3SSs) are present in several pathogenic bacteria (Viprey et al, 1998; Cornelis, 2002). Type III secretion systems present T3SS-dependent extracellular appendages that link bacteria to their hosts (Saad et al, 2008). In animal pathogens these appendages are called needle structures. Flavonoids and NodD induce the expression of rhizobial T3SS components and effectors since the gene encoding the transcriptional factor TtsI contains a nod box consensus sequence in its promoter region (Krause et al, 2002; Marie et al, 2004). Genes that code for proteins secreted by this system in M. loti and with functionality in nodulation competitiveness (mlr6316, mlr6331, mlr6361, and mlr6358) were localized in the symbiotic island, outside of the T3SS cluster (Hubber et al, 2004; Sánchez et al, 2009, 2012)
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