Substrate selectivities of microbial proline 4-hydroxylase and proline 3-hydroxylases, all of which were purified from recombinant Escherichia coli, were investigated. L-2-Azetidine carboxylate, 3,4-dehydro-L-proline and L-pipecolinic acid were hydroxylated by those enzymes in regio- and stereospecific manner. (C) 1999 Elsevier Science Ltd. All rights reserved.