SUMMARYThe Mediator complex is commonly seen as a molecular bridge that connects DNA-bound transcription factors to the RNA polymerase II (Pol II) machinery. It is a large complex of 30 subunits that is present in all eukaryotes. The Med12 subunit has been implicated not only in the regulation of Pol II activity, but also in the binding of transcription factors to the bulk of the Mediator complex. We targeted Med12 in mouse embryonic stem cells to investigate the in vivo function of this subunit. We report here the developmental defects of Med12 hypomorphic mutants that have a drastic reduction in Med12 protein levels. These mutants fail to develop beyond embryonic day 10 and have severe defects in neural tube closure, axis elongation, somitogenesis and heart formation. We show that in Med12 hypomorphic embryos, the Wnt/planar cell polarity pathway is disrupted and that canonical Wnt/b-catenin signaling is impaired. In agreement with this, embryos that are incapable of Med12 expression failed to establish the anterior visceral endoderm or activate brachyury expression, and did not complete gastrulation.

e Enteropathogenic Escherichia coli (EPEC) is recognized as an important intestinal pathogen that frequently causes acute and persistent diarrhea in humans and animals. The use of probiotic bacteria to prevent diarrhea is gaining increasing interest. The probiotic E. coli strain Nissle 1917 (EcN) is known to be effective in the treatment of several gastrointestinal disorders. While both in vitro and in vivo studies have described strong inhibitory effects of EcN on enteropathogenic bacteria, including pathogenic E. coli, the underlying molecular mechanisms remain largely unknown. In this study, we examined the inhibitory effect of EcN on infections of porcine intestinal epithelial cells with atypical enteropathogenic E. coli (aEPEC) with respect to single infection steps, including adhesion, microcolony formation, and the attaching and effacing phenotype. We show that EcN drastically reduced the infection efficiencies of aEPEC by inhibiting bacterial adhesion and growth of microcolonies, but not the attaching and effacing of adherent bacteria. The inhibitory effect correlated with EcN adhesion capacities and was predominantly mediated by F1C fimbriae, but also by H1 flagella, which served as bridges between EcN cells. Furthermore, EcN seemed to interfere with the initial adhesion of aEPEC to host cells by secretion of inhibitory components. These components do not appear to be specific to EcN, but we propose that the strong adhesion capacities enable EcN to secrete sufficient local concentrations of the inhibitory factors. The results of this study are consistent with a mode of action whereby EcN inhibits secretion of virulenceassociated proteins of EPEC, but not their expression.