Section 4 — Gene Action 



4.43. Structural Gene for Ornithine Transcarbamy- 

 lase in Neurospora. R. H. Davis and W. M. 

 Thwaites (Ann Arbor, U.S.A.). 



Previous workers have been unable to isolate 

 mutants of Neurospora lacking the enzyme or- 

 nithine transcarbamylase (OTC), which cata- 

 lyzes the conversion of ornithine to citrulline in 

 the metabolic pathway leading to arginine. The 

 mutation s, recognizable by its effect upon pyri- 

 midine- and prolinere-quiring phenotypes but not 

 by a nutritional requirement, has been shown to 

 contain 3 percent of normal OTC activity (1) . 

 Using conidia of the s strain, many OTC-less 

 mutants were isolated after u.v. irradiation. They 

 were non-complementing, and segregated as single 

 gene mutations when mated with wild type; 

 s was not recovered in such crosses. Extracts of 

 an OTC-less mutant did not interfere with OTC 

 activity of wild or 5 extracts. The locus repre- 

 sented by 5 and OTC-less mutants is denoted 

 arg-12 and is concluded to be the single structural 

 gene for OTC. The locus is genetically distinct 

 from mutations at the arg-2 and arg-3 loci which, 

 like arg-12 mutants, are stimulated by citrulline 

 and arginine but not ornithine. A similar search 

 for OTC-less mutants starting with wild type 

 conidia yielded only one OTC-less mutant, which 

 was allelic with s. The difficulty in isolating 

 mutants of this class from wild type may be due 

 to a high OTC content of conidia formed by 

 wild type. 



Research supported by U.S. National Science 

 Foundation. 



1. Davis, Genetics 47, 351-360, 1962. 



4.44. Ornithine Transcarbamylaseless Mutants of 

 Neurospora. V. W. Woodward (Houston, 

 U.S.A.). 



The reaction coupling carbamyl phosphate 

 with ornithine to yield citrulline is catalyzed by 

 the transferase, ornithine transcarbamylase 

 (OTC). The genetic control of this enzyme has 

 been obscured by the finding that mutants arg-2 

 and arg-3, mutants heretofore assumed to be 

 "blocked" between ornithine and citrulline, con- 

 tain wild-type amounts of OTC, and by the dis- 

 covery that the suppressor mutation, s, which 

 relieves certain pyr-3 mutants of their pyrimidine 

 dependence, contains no more than 3 per cent 

 of wild-type OTC activity. Attempts to isolate 

 additional mutants lacking OTC have taken two 

 courses: (1) since s (reduced OTC) suppresses 

 certain pyr-3 mutants, a search was made for 



further suppressors. Many OTC-less mutants 

 were found, all of which require arginine, or 

 citrulline, but none of which suppress pyr-3, 

 despite the fact that the pyr-3 OTC-less double 

 mutants were isolated on minimal medium. The 

 OTC-less mutants are not allelic with arg-2 or 

 arg-3. (2) Using 5 as the conidial source, the 

 filtration and selective plating technique was used 

 to isolate arginine-s double mutants. From such 

 double mutants six of the known arginine- 

 requiring mutants and two OTC-less mutants 

 were recovered. Physiological and genetical 

 properties of these mutants will be discussed. 



Research supported by grant number RG- 

 8653, National Institutes of Health. 



4.45. Quantitatively and Qualitatively Altered Phenol- 

 oxydases in Podospora anserina, due to Mu- 

 tations at Non-linked Loci. Karl Esser (Co- 

 logne, Germany). 



Wild strains of the ascomycete Podospora 

 anserina produce melanin pigments on suitable 

 media. The early steps in the formation of these 

 pigments are catalysed by phenoloxydases. Two 

 different phenoloxydases have been found in the 

 wild strain si-: a diphenoloxydase (laccase) and 

 a monophenoloxydase (tyrosinase). The latter is 

 formed in only very small amounts compared to 

 laccase. Both enzymes can be distinguished by 

 their substrate specifities. 



During the past years we have obtained about 

 40 more or less pigmentless mutants, most of 

 which have occurred spontanously. By 5-6 sub- 

 sequent backcrosses with wild strain si-, these 

 mutants have been made highly isogenic. Some 

 of the mutants are allelic. They all differ from 

 the wild strain by a single gene and could be 

 localized in 5 of the 7 linkage groups. With these 

 deficiences in pigment formation concomitant 

 defects in fertility are always observed. The 

 normal cycle of development of the mutants is 

 blocked at different stages. 



Since all of the mutants exhibit different de- 

 grees of phenoloxydase activity, we wanted to 

 know whether this is due to quantitative or to 

 qualitative alterations of the enzymes. In order 

 to distinguish between the two alternatives, we 

 have performed a preliminary serological test: 

 enzyme extracts of the mutants were assayed for 

 cross reaction with antibody against partially 

 purified wild type enzymes (CRM test). Some of 

 the mutants showed considerable deviations of 

 the Enzyme/CRM ratio which is 1 for the wild 

 strain. Therefore, the concerned enzymes seem 



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