RECURRENT MUTATION-SELECTION TO IMPROVE RENNET PRODUCTION IN CANDIDA-TSUKUBAENSIS

The yeast Candida tsukubaensis is of industrial importance for the pro duction of microbial milk-clotting enzyme. Milk-clotting enzyme is an enzymatic complex capable of coagulating milk for cheese manufacturing . High clotting activity (CA) and low proteolytic activity (PA) are de sirable qualities. To study the genetic nature of the CA and PA traits , we analyzed 179 colonies obtained after mutagenic treatment. Analysi s of the data obtained for this populations showed that CA and PA are traits controlled by polygenes and that they are correlated (r = 0.3). The existence of a positive correlation indicates that selection for one trait without considering the other may alter one of the traits in an undesirable direction, since the objective of selection would be a n increase in clotting activity and a decrease in proteolytic activity . Three cycles of recurrent mutation-selection were carried out to obt ain improved strains. The ultraviolet light dose permitting a 5% rate of cell survival was sufficient to generate genetic variability in the three selection cycles. At the end of the third cycle there was an in crease of about 98% in clotting activity and a decrease of about 20% i n proteolytic activity. Analysis of variance of the selective cycles s howed that the linear effects were significant (P < 0.01) for both tra its. Estimates of genetic variances and heritabilities of the three se lection cycles are presented.