Growth of yersiniae is restricted at 37 degrees C in the absence of calcium ions. This phenomenon correlates with the massive release of a set of proteins called Yops. Growth restriction and Yops production are governed by a 70 kb plasmid called pYV. yop genes are distributed throughout pYV and constitute a thermoactivated regulon controlled by the gene virF. The transcription activator VirF is a member of a new family of regulators including those of the arabinose and rhamnose operons as well as a regulator of enteric colonization pili. The role of calcium ions on the release of Yops remains largely unknown.