A series of translational fusions between the Yersinia pseudotuberculosis inv locus and lacZ was constructed. Each Lac+ fusion strain expressed a hybrid protein containing invasin, the product of the inv locus, at its amino-terminal end. Analysis of these gene fusions allowed determination of the direction of translation of the inv gene. Previous studies of Y. pseudotuberculosis invasion have shown that entry into animal cells is temperature regulated. It is shown here that control of expression of the inv gene is also temperature regulated. phoA gene fusions to inv, when present in Y. pseudotuberculosis, were expressed at lower levels when bacteria were grown at 37 degrees C rather than at 28 degrees C. Similar fusions, in contrast, were regulated in a temperature-independent fashion in Escherichia coli, as was the wild-type inv gene. This implies that Y. pseudotuberculosis has chromosomally encoded trans-acting functions that normally thermoregulate expression of inv.