Expression of six peptidases from Lactobacillus helveticus in Lactococcus lactis

For development of novel starter strains with improved proteolytic properti es, the ability of Lactococcus lactis to produce Lactobacillus helveticus a minopeptidase N (PepN), aminopeptidase C (PepC), X-prolyl dipeptidyl aminop eptidase (PepX), proline iminopeptidase (PepI), prolinase (PepR), and dipep tidase (PepD) was studied by introducing the genes encoding these enzymes i nto L. lactis MG1363 and its derivatives. According to Northern analyses an d enzyme activity measurements, the L. helveticus aminopeptidase genes pepN , pepC, and pepX are expressed under the control of their own promoters in L. lactis, The highest expression level, using a low-copy-number vector, wa s obtained,vith the L. helveticus pepN gene, which resulted in a 25-fold in crease in PepN activity compared to that of wild-type L. lactis, The L. hel veticus pepI gene, residing as a third gene in an operon in its host, was e xpressed in L. lactis under the control of the L. helveticus pepX promoter. The genetic background of the L. lactis derivatives tested did not affect the expression level of any of the L. helveticus peptidases studied. Howeve r, the growth medium used affected both the recombinant peptidase profiles in transformant strains and the resident peptidase activities. The levels o f expression of the L. helveticus pepD and pepR clones under the control of their own promoters were below the detection limit in L. lactis, However, substantial amounts of recombinant pepD and PepR activities were obtained i n L. lactis when pepD and pepR were expressed under the control of the indu cible lactococcal nisA promoter at an optimized nisin concentration.