Genetic studies on temperature-sensitive mutants of Lactobacillus casei phage J1 (author's transl)

Abstract

Temperature-sensitive mutants (ts mutants) of Lactobacillus casei phage J1 were isolated by treatment with nitrous acid or hydroxylamine. A total of 56 ts mutants were classified into 9 groups, I-IX, by the complementation test. The representative ts mutants of each group were assigned on a linear map according to the additive recombination frequency in two factor crosses.In temperature-shift experiments, the ts mutants of groups II and III were shown to be defective in the early functions for phage multiplication. Groups I, VI, VIII and IX were defective in the middle functions, whereas groups, IV, V, and VII were defective in the late functions.The formation of serum-blocking power could be detected with the ts mutants of groups I, IV, V, VII, and VIII, but not with those of groups, II, III, and VI.The synthesis of phage-specific DNA was measured by the incorporation of 3H-thymidine into an acid-insoluble fraction using the host bacteria whose DNA synthesis had been arrested by ultraviolet irradiation. The mutants of groups II and III were found to be incapable of synthesizing phage-specific DNA at the nonpermissible temperature.On the basis of genetic analysis and findings on gene expressions, the ts mutants defective in the early functions (groups II and III) appeared to be located near one end of the genetic map.