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Isolation and Partial Characterization of a Particulate Proteinase from a Slow Acid Producing Mutant of Streptococcus lactis

Department of Food Science North Carolina State University, Raleigh 27607

ABSTRACT

About 1.2% of the colonies isolated from plates of the parent Streptococcus lactis No. 3 were slow acid producing mutants. One was selected for further study. The mutant required approximately 72 hr at 22 C to produce sufficient acid to coagulate 11.0% nonfat milk and was unable to obtain a final population of more than 2 x 10⁸ cells per milliliter in milk. Growth studies suggested that the mutant's proteinase system was inadequate. However, the proteinase isolated from the particulate fraction of disrupted cells of the mutant was similar to the membrane-associated proteinase of the parent strain. Both enzymes had the same pH for maximal activity toward casein, electrophoretic migration distances, and substrate specificity on alpha_s1-casein.

¹ Present address: Department of Dairy Science, University of Maryland, College Park, Maryland 20742.

² Present address: Dental Research Unit, Veterans Administration Hospital and Institute of Oral Biology, University of Miami, Miami, Florida 33125.