Upon incubation at 37°C in the absence of Ca2+ ions, pathogenic yersiniae release high amounts of pYV plasmid-encoded proteins called Yops, involved in pathogenesis. Yersinia enterocolitica also express two outer membrane proteins, an adhesin called YadA and a lipoprotein called YIpA. The production of the Yops is co-ordinately regulated by a 20kb region of the plasmid referred to as the 'Ca2+ dependence region' and containing at least four loci called virA, virB, virC, and virF. The 8.5kb virC region, involved in the specific transport of the Yops, is a single operon containing 13 open reading frames called yscA to yscM. Gene virF encodes a key transcriptional activator of the yop, yadA and ylpA genes. It is only transcribed at 37°C and its expression is modulated by a chromosome-encoded histone-like protein called YmoA. We show here that virF also controls the virC operon. By contrast, VirF is not essential for the induction of virA and virB. The VirF protein binds specifically to yop promoters. In particular, it protects the region spanning nucleotides -64 to -34 of yopH. In order to analyse the role of temperature in the induction of the yop regulon, we constructed Y. enterocolitica strains expressing virF from the tac promoter. In spite of the fact that virF was transcribed at 25°C, neither the Yops nor YadA were expressed at that temperature. This poor response to VirF at 25°C was at least partially due to a weak and slow transcription of the genes controlled by virF. Surprisingly, when cloned on pACYC184, gene yadA was expressed even in absence of VirF, but remained thermodependent. Hence temperature and virF are both required for the induction of the yop regulon. Among other possible roles, temperature could modify the structure of either the activator itself or the yop promoter. The fact that VirF binds in vitro to yop promoters at 25°C rules out the first hypothesis. In order to test the second hypothesis, we studied, in vivo, the activity of the yopH promoter in ymoA mutants. The yopH promoter became active in the absence of VirF, indicating that yop promoter activity depends upon chromatin structure. We conclude from these two observations that, in vivo, temperature is required to modify the DNA structure of the yop promoters in order to allow the action of the transcriptional activator.