| Summary: | <p>Abstract</p> <p>Background</p> <p>Pathogenic yersiniae (<it>Y. pestis</it>, <it>Y. pseudotuberculosis</it>, <it>Y. enterocolitica</it>) share a virulence plasmid encoding a type three secretion system (T3SS). This T3SS comprises more than 40 constituents. Among these are the transport substrates called Yops (<it>Yersinia </it>outer proteins), the specific Yop chaperones (Sycs), and the Ysc (Yop secretion) proteins which form the transport machinery. The effectors YopO and YopP are encoded on an operon together with SycO, the chaperone of YopO. The characterization of SycO is the focus of this study.</p> <p>Results</p> <p>We have established the large-scale production of recombinant SycO in its outright form. We confirm that <it>Y. enterocolitica </it>SycO forms homodimers which is typical for Syc chaperones. SycO overproduction in <it>Y. enterocolitica </it>decreases secretion of Yops into the culture supernatant suggesting a regulatory role of SycO in type III secretion. We demonstrate that <it>in vitro </it>SycO interacts with YscM1, a negative regulator of Yop expression in <it>Y. enterocolitica</it>. However, the SycO overproduction phenotype was not mediated by YscM1, YscM2, YopO or YopP as revealed by analysis of isogenic deletion mutants.</p> <p>Conclusion</p> <p>We present evidence that SycO is integrated into the regulatory network of the <it>Yersinia </it>T3SS. Our picture of the <it>Yersinia </it>T3SS interactome is supplemented by identification of the SycO/YscM1 interaction. Further, our results suggest that at least one additional interaction partner of SycO has to be identified.</p>
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