Genetic Determination of Tyrosinase Thermostability

(HOROWITZ and SHEN 1952). The strain produces almost no tyrosinase activity when cultured at 35" C on a medium which favors the production of strong activity in 25" cultures. The evidence indicated that the temperature effect is not due to formation of a tyrosinase inhibitor, but to a net decrease in tyrosinase synthesis at the higher temperature. Attention has been called to the resemblance between this case and that of the Himalayan rabbit, in which the formation of melanin, the end-product of tyrosinase activity, fails to take place at body temperature (HOROWITZ 1951). On the basis of the analogy with Himalayan, a search was made for strains in which tyrosinase is produced at both 25" and 35" (analogous with selfcolored) and strains in which the enzyme is produced at neither temperature (analogous with albino). A number of strains analogous with self-colored, in the abovq sense, were found. When the tyrosinase in crude extracts of one of these strains was tested, it was found to be much more stable to heat than is the tyrosinase obtained from strain 4A. This discovery led us to an investigation of the following points : (a) the mode of inheritance of the thermostability, (b) the chemical basis of the difference in thermostability, and (c) the relationship between thermostability and enzyme production at 35". I' 4A of Neurospora c-yL1ssa is influenced by the temperature of cultivation