This study describes the involvement of the sigma factor of the flagellar system, FliA, in global gene regulation of Yersinia enterocolitica. In addition to exhibiting a positive effect upon the expression levels of eight class III flagellar operons, FliA also exhibited a negative effect upon the expression levels of four virulence operons that are located on the pYV virulence plasmid. These are yadA, virC, yopQ, and the insertion element ISYen1. While the positive effect on class III flagellar operons by FliA is most likely direct, the negative effect on the virulence operons appears to require the known transcriptional activator of these genes, VirF. This was determined using microarray analysis, quantitative PCR and a search for putative binding sites for FliA. In addition to the FliA regulation of flagellar and plasmid-encoded virulence genes, we studied temperature regulation of these genes. While wild-type cells exhibited increased expression levels of flagellar genes and decreased expression levels of plasmid-encoded virulence genes at 25 degrees C (as compared to 37 degrees C), temperature dependence of gene expression was much reduced in the fliA mutants. We conclude that FliA contributes to the inverse temperature regulation of flagellar and plasmid-encoded virulence genes. We present a network of transcriptional regulation around FlhD/FlhC and FliA.