The DNA coding for pro-papaya proteinase IV (PPIV) has been cloned and expressed in Escherichia coli. Heterologous expression of the protein, followed by refolding in vitro, yields an enzymatically active pro-enzyme which fails to autodigest to form the mature protein. Mutagenesis of the active site of papain to simulate that of PPIV yields a proenzyme which also fails to autoactivate. Complementary mutagenesis of the pro-region/mature boundary of PPIV, to introduce its own substrate recognition sequence, has, however, produced a pro-enzyme that will autocatalytically cleave. This is the first report of enzymatic activity in a recombinant pro-cysteine proteinase, and the first time that such a protein has been shown to fail to autocatalytically cleave because of its stringent substrate specificity.