The peptidyl prolyl cis-trans isomerase (PPIase, EC 5.2.1.8) gene (ppiT) from Bacillus stearothermophilus SIC1 was cloned on the basis of a partial amino acid sequence of the purified enzyme. ppiT was found as an open reading frame (501 bases) which coded for a protein consisting of 167 amino acid residues (molecular weight, 18,349) (GenBank accession number D42050). The cloned ppiT was overexpressed in Eschelichia coil cells using pET-8c as an expression vector. The enzyme was purified by heat treatment and column chromatography on DEAE-Sepharose CL-6B. Purification was about 148-fold and the molecular weight of the enzyme was estimated to be about 18.0 kDa by SDS-PAGE. PPIase activity was determined using synthetic peptide as a substrate in a 2-step reaction coupled with chymotrypsin treatment. The enzyme was stable at pH 7.5-8.0. No heat denaturation was observed when the enzyme was treated at 60 degrees C for 30 min. The PPIase purified from recombinant E. coil has almost the same characteristics as that from B. stearothemophilus SIC1. In refolding solution, the PPIase enhanced the Isomerization rate of unfolded RNase T1.