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Background: Pathogenic yersiniae (Y. pestis, Y. pseudotuberculosis, Y. enterocolitica) 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 (Yersinia 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. Results: We have established the large-scale production of recombinant SycO in its outright form. We confirm that Y. enterocolitica SycO forms homodimers which is typical for Syc chaperones. SycO overproduction in Y. enterocolitica decreases secretion of Yops into the culture supernatant suggesting a regulatory role of SycO in type III secretion. We demonstrate that in vitro SycO interacts with YscM1, a negative regulator of Yop expression in Y. enterocolitica. However, the SycO overproduction phenotype was not mediated by YscM1, YscM2, YopO or YopP as revealed by analysis of isogenic deletion mutants. Conclusion: We present evidence that SycO is integrated into the regulatory network of the Yersinia T3SS. Our picture of the Yersinia 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. Background An ever increasing number of Gram-negative bacteria is known to utilize a type three secretion system (T3SS), mostly pathogens that manipulate their host cells by injecting effector proteins [1]. One of the characteristics of T3SSs is the existence of specific energy-independent chaperones which assist several of the transport substrates. Chaperones assisting the effectors (class I chaperones) are distinguished from those assisting the pore forming translocators (class II chaperones) [2]. The mode Published: 5 July 2007 BMC Microbiology 2007, 7:67 doi:10.1186/1471-2180-7-67 Received: 29 November 2006 Accepted: 5 July 2007 This article is available from: http://www.biomedcentral.com/1471-2180/7/67 © 2007 Dittmann et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.