Virulent Y. enterocolitica strains restrict their growth at 37 degrees C, in rich medium deprived of calcium. This property, called calcium dependency, correlates with pathogenicity. It is conditioned by a 70 kb plasmid called pYV. The analysis of calcium independent (Cl) insertion mutants defined a 20 kb region called the calcium region. This region contains at least three transcription units called virA, B and C. In growth restriction conditions, Y. enterocolitica releases and inserts in its outer-membrane several pYV encoded proteins (POMPs). By transposition mutagenesis using a mini-Mu dlac element, we localized plasmid genes encoding POMPs of 84, 51, 30 and 25 kDa. These genes appear to be scattered on pYV, outside the calcium region and to constitute a regulon. Transcription of these yop genes increased by factors of 3.5 (yop84) to 200 (yop51) when temperature was shifted from 25 to 37 degrees C. On the contrary, calcium had only a moderate effect. Transcription was also dependent on the culture medium and on a trans-acting factor encoded by the calcium region. One Cl double mutant severely affected in transcription of yop51 defined a new locus called virF in the calcium region of pYV. Transcription of the yop genes was very poor in E. coli K12, even in the presence of a fully functional calcium region.