Molecular cloning, sequencing, and mapping of the gene encoding protease I and characterization of proteinase and proteinase-defective Escherichia coli mutants

Abstract

Clones carrying the gene encoding a proteinase were isolated from Clarke and Carbon's collection, using a chromogenic substrate, N-benzyloxycarbonyl-L-phenylalanine beta-naphthyl ester. The three clones isolated, pLC6-33, pLC13-1, and pLC36-46, shared the same chromosomal DNA region. A 0.9-kb Sau3AI fragment within this region was found to be responsible for the overproduction of the proteinase, and the nucleotide sequence of the region was then determined. The proteinase was purified to homogeneity from the soluble fraction of an overproducing strain possessing the cloned gene. N-terminal amino acid sequencing of the purified protein revealed that the cloned gene is the structural gene for the protein, with the protein being synthesized in precursor form with a signal peptide. On the basis of its molecular mass (20 kDa), periplasmic localization, and substrate specificity, we conclude this protein to be protease I. By using the gene cloned on a plasmid, a deletion mutant was constructed in which the gene was replaced by the kanamycin resistance gene (Kmr) on the chromosome. The Kmr gene was mapped at 11.8 min, the gene order being dnaZ-adk-ush-Kmr-purE, which is consistent with the map position of apeA, the gene encoding protease I in Salmonella typhimurium. Therefore, the gene was named apeA. Deletion of the apeA gene, either with or without deletion of other proteinases (protease IV and aminopeptidase N), did not have any effect on cell growth in the various media tested.