Section 4 — Gene Action 



linked to the structural gene (T) of tyrosinase. 

 The induction is pH dependent. It occurs opti- 

 mally at pH 6 and is inhibited at pH 8. Inhibition 

 at pH 8 is due to the loss of amino acids and 

 calcium from the cells. The induction is inhibited 

 only slightly by actinomycin-D, a drug which 

 blocks DN A-dependent RN A synthesis and which 

 strongly inhibits the growth of Neurospora. This 

 result suggests that messenger-RNA synthesis 

 for tyrosinase occurs during the period of active 

 growth, but is not used for tyrosinase synthesis 

 while growth is occurring. 



1. See Cold Spring Harbor Symp. Quantitative 

 Biol. 26, 233, 1961. 



4.35. Comparative Genetic Studies of Tyrosinase 

 Synthesis in Neurospora crassa and Bacillus 

 subtilis. K. E. Fuscaldo, V. Del Vecchio and 

 W. Kaczmarczyk (New York, U.S.A.). 



The genetic control of tyrosinase production in 

 Neurospora crassa is being studied. Two strains, 

 15300-1 31a and 15300- 138 A, had been isolated 

 by other workers and characterized for their 

 ability to produce tyrosinases. Three electro- 

 phoretically separable enzymes'were found which, 

 however, do not differ with respect to their 

 Michaelis constants or relative turnover numbers. 

 It appears from previous work that the three 

 forms of the enzyme were present in homoca- 

 ryons. In order to clarify this system, matings 

 were made between the two strains and extensive 

 tetrad analyses carried out to establish the mode 

 of transfer of the genetic information regulating 

 tyrosinase synthesis. It has been suggested that 

 the T locus in Neurospora controls the equili- 

 brium among several interconvertible tyrosinases 

 and therefore is concerned with the terminal 

 stages of tyrosinase synthesis (Fox et al.). Other 

 workers have interpreted their results to con- 

 clude that the T locus determines the structure of 

 the enzyme and two other loci, ty-1 and ty-2, 

 exert a regulatory control on the tyrosinase 

 system (Horowitz et al.). 



A corollary study was undertaken to examine 

 the genetics of tyrosinase production in another 

 organism. B. subtilis was chosen for this investi- 

 gation since it had been reported that a variety 

 (niger) produced a black pigment when grown 

 on tyrosine supplemented media. The decision 

 to pursue the tyrosinase studies on this organism 

 was predicated on the fact that bacteria through 

 their ability to participate in the phenomena of 

 transduction and transformation offered ex- 

 cellent tools for the genetic analysis' of an appa- 



rently complex system. Further, since these or- 

 ganisms possess a "chromosome" which is 

 relatively simple when compared to those found 

 in higher organisms, it may be possible to dis- 

 tinguish the two control mechanisms on that 

 basis. 



4.36. A Number of Loci associated with the Reduction 

 of Nitrate to Nitrite in Aspergillus nidulans. 

 B. M. Rever and D. J. Cove (Cambridge, 

 Great Britain). 



In a sample of 1200 mutant strains produced 

 by u.v. irradiation of conidia of a prototrophic 

 strain of the fungus Aspergillus nidulans, 40 were 

 unable to grow on nitrate, but were able to use 

 nitrite as a nitrogen source. Pairs of mutant 

 strains were tested for complementary growth in 

 heterokaryons. On this basis, the mutants fell 

 into nine separate classes. The study of diploid 

 growth complementation has now begun. The 

 results of preliminary experiments suggest that 

 there may be a difference between heterokaryon 

 and diploid complementation with respect to 

 certain pairs of mutants. 



It is known that all forty mutants lack normal 

 nitrate reductase activity. In the few cases so far 

 investigated, complementary heterokaryons and 

 diploids show detectable nitrate reductase 

 activity. 



Linkage studies on representatives of the nine 

 complementation groups indicate that at least 

 six separate loci are involved, none of which is 

 closely linked to another. The loci may be allocat- 

 ed to their linkage groups using haploidization 

 techniques. 



A discussion of these results will be made in 

 conjuction with the discussion of the biochemical 

 aspects of this system in our later paper. 



4.37. Biochemical Genetical Studies on Nitrate 

 Reduction in Aspergillus nidulans. D. J. Cove 

 and B. M. Rever (Cambridge, Great Britain). 



The reduction of nitrate to nitrite is thought, 

 in many organisms, to be catalysed by a single 

 enzyme, nitrate reductase. This enzyme activity 

 has been detected in extracts of mycelium of 

 prototrophic strains of Aspergillus nidulans, and 

 can be assayed quantitatively, in two ways. The 

 rate of oxidation of NADPH2, with which the 

 enzyme is linked, may be observed spectrophoto- 

 metrically, or the nitrate produced may be esti- 

 mated colorimetrically. The enzyme activity is 

 inducible. 



48 



