Molecular cloning and DNA sequence analysis of pepL, a leucyl aminopeptidase gene from Lactobacillus delbrueckii subsp. lactis DSM7290

Abstract

A genomic library of Lactobacillus delbrueckii subsp. lactis DSM7290 DNA fragments from a Sau3A partial digestion in the low-copy-number vector pLG339, was used to screen Escherichia coli for the presence of peptidases. Using the chromogenic substrate leucine-beta-naphthylamide (Leu-NH-Nap) and E. coli strain CM89 lacking the corresponding enzyme activity in an enzymic plate assay, allowed the isolation of two peptidase genes; the newly described pepL and the recently cloned and sequenced pepN. Clones could be distinguished not only by the restriction pattern of isolated plasmids but also by the rate and intensity of their colour reaction with Leu-NH-Nap. Three out of five clones were identified to express the Lactobacillus pepN gene; the others were shown to express a second aminopeptidase gene, designated pepL. This gene, together with 200 bp upstream of the proposed AUG initiation codon, was further subcloned and sequenced. The corresponding open reading frame of 897 nucleotides is predicted to encode a protein of 299 amino acids (34,541 Da). Searching the EMBL database revealed similarity to the prolinase of Lactobacillus helveticus (45.8% identity), to the iminopeptidases of Lb. delbrueckii subsp. lactis and Lb. delbrueckii subsp. bulgaricus (25.5%), and to the Bacillus coagulans prolinase (21.5%). Minor similarities were detected for hydrolytic enzymes with serine active sites. The product encoded by the pepL gene was functional but could not be visualized on Coomassie-blue-stained polyacrylamide gels. High level expression of peptidase L in E. coli was achieved by placing the gene under the control of the T7 promoter.