The effects of genetic changes on tyrosinase activity in Glomerella.

N E of the most significant and interesting problems in biology concerns 0 the relationship between genes and enzymes. Recently considerable attention has been directed towards the hypothesis of genic action which states that the primary function of a gene lies in the control of enzyme synthesis (HOROWITZ 1949; BEADLE 1945, 1948; HALDANE 1942; WRIGHT 1941). I n support of this concept several investigators have established correlations between the activityof an enzyme and the genetic constitution of an organism.However, the particular kind of correlation established varies with the different investigations. For example, MITCHELL and LEIN (1948) have shown that a single gene change completely prevents any measurable activity of the enzyme which catalyzes the coupling of serine and indole to form tryptophane in Neurospora. Similarly, SAWIN and GLICK (1943) working with atropinesterase from the blood serum of rabbits showed that this enzyme was inherited as an incompletely dominant trait. Homozygous recessive rabbits produced serum with no atropinesterase activity while the serum from the heterozygotes had about half the enzymatic activity of the serum from the dominant homozygotes. Thus, in this case, a single gene seems to exert an effect on the quantity of an enzyme produced. On the other hand, the work of WINGE and ROBERTS (1948) on maltose fermentation by Saccharomyces cerevisiae indicates a polygenic control of enzyme synthesis. They found that any one of three non-allelic genes enabled S. cerevisiae to ferment maltose, presumably by bringing about the synthesis of maltase. A somewhat different kind of polygenic control of an enzyme, involving the degree of enzyme activity, is shown by.the investigations of RUSSELL and RUSSELL (1948) on the intensity of dopa oxidase activity in the hair bulbs of the skin of mice. The dopa oxidase activity was found to be proportional to the intensity of yellow pigment in the hair and to be under the control of several different genes. These examples of the genetic control of enzyme synthesis all involve measurements of the degree of enzymatic activity. Such differences in enzymatic activity may be attributed either to differences in the amount of the enzyme present or to different efficiencies of the same quantity of enzyme. Different efficiencies may in turn be explained by associated changes of the chemical milieu (pH changes, for example) in which the enzyme is acting or to inherent differences in the enzyme molecule itself. It is this last interpretation which forms the basis of the hypothesis (cf. BEADLE 1945) that the gene determines the specificity of the enzyme. Some of the support for this hypothesis stems