Summary -Dairy starter strains Streptococcus thermophilus CNRZ 385 and CNRZ 703 coagulatad low heat skim milk in 3 h while the raference strain CNRZ 302 required more than 10 h. The high acidification rates of these 2 strains (H-strains) were correlated with the presence of a 10-and 7-fold increase in proteinase activity as compared to 13 other randomly chosen strains (L-strains), including the reference strain CNRZ 302. The level of proteinase activity in M17 broth was similar ta the activity of the Prt+ Lactococcus lactis ssp lactis strain CNRZ 1076. Proteinase activity of S thermophilus H-strains was found ta be cell wall-associated and was not released in the absence of CaCI 2 as in the case of L lactis strains. The relevance of these proteinase activities in relation to high acid production rates was confirmed by a proteinase-negative mutant whose growth and proteinase activity were reduced to levels observed for the L-strains. Streptococcus

The production and biochemical properties of cell envelope-associated proteinases from two strains of Streptococcus thermophilus (strains CNRZ 385 and CNRZ 703) were compared. No significant difference in proteinase activity was found for strain CNRZ 385 when cells were grown in skim milk medium and M17 broth. Strain CNRZ 703 exhibited a threefold-higher proteinase activity when cells were grown in low-heat skim milk medium than when grown in M17 broth. Forty-one percent of the total activity of CNRZ 385 was localized on the cell wall. The optimum pH for enzymatic activity at 37°C was around 7.0. Serine proteinase inhibitors, such as phenylmethylsulfonyl fluoride and diisopropylfluorophosphate, inhibited the enzyme activity in both strains. The divalents cations Ca2+, Mg2+, and Mn2+ were activators, while Zn2+ and Cu2+ were inhibitors. D-Casein was hydrolyzed more rapidly than q.1-casein. The results of DNA hybridization and immunoblot studies suggested that the S. thermophilus cell wall proteinase and the lactococcal proteinase are not closely related.