title: Cloning of the pepX gene of Lactobacillus helveticus IF03809 encoding salt-tolerant X-prolyl dipeptidyl aminopeptidase and characterization of the enzyme.
abstract: X-prolyl dipeptidyl aminopeptidase (X-PDAP) from Lactobacillus helveticus IF03809 expressed nearly full activity under high salt conditions, such as 2 M NaCl. We cloned and sequenced the pepX gene for X-PDAP. The calculated M, of deduced X-PDAP (803 amino acids) was 90,847 and the protein was distantly related (35 to 44% identity) to known X-PDAPs of Lactobacillus sp. including L. helveticus CNRZ32 (40% identity). Native and recombinant X-PDAP were purified to homogeneity from both L. helveticus IF03809 and Escherichia coli DH5alpha harboring the pepX gene on a plasmid, respectively. The native enzyme appeared to be a dimer of 220 kDa, as estimated by gel filtration column chromatography. It hydrolyzed an X-prolyl-linkage, but not prolyl- or X-prolyl-X-peptide bonds, and tolerated up to 2 M NaCl as well as some other chlorides of monovalent cations. Determination of the flanking sequences revealed two divergent genes. The upstream region of the pepX gene encodes oppA gene for a putative oligopeptide permease, while the downstream region encodes tnp gene specifying a possible transposase of the IS3 family. The oppA gene shares a 176 bp-promoter region with pepX in the intergenic region, implying a relationship between this oligopeptide transport system and X-PDAP.
authors.affiliation: Division ofApplied Microbiology, National Food Research Institute, 2-1-12 Kannondai, Tsukuba, Ibaraki 305-8642, Japan.
authors.forename: Keitarou
authors.initials: K
authors.lastname: Kimura
authors.forename: Atsushi
authors.initials: A
authors.lastname: Nagasawa
authors.forename: Mikio
authors.initials: M
authors.lastname: Fujii
authors.forename: Yoshifumi
authors.initials: Y
authors.lastname: Itoh