Section 4 — Gene action 



growth of unmodified mutants is inhibited sev- 

 erely at high concentrations. 



4.49. Genetic and Biochemical Studies of Cytochrome 

 C deficient Mutants of Yeast. Fred Sherman 

 and Harry Taber (Rochester, U.S.A.). 



In a previous investigation, a single-gene mu- 

 tant of yeast was found which had a decreased 

 concentration of cytochrome c. The cytochrome 

 c isolated from this mutant (cyi-i) was different 

 from wild type (cyi-i) cytochrome c by a number 

 of criteria. W 



A rapid method was developed for examining 

 the cytochrome spectrum of yeast strains at 

 -190°C in order to obtain other mutants con- 

 trolling cytochrome c. After examining over 

 14,000 strains that were treated with either ultra- 

 violet light or nitrous acid, ten mutants were 

 isolated that had a decreased amount of cyto- 

 chrome c but normal or near normal amounts of 

 cytochromes a and b (defined as cy). All ten 

 mutants were crossed to wild type (CY) and the 

 original cyi-i mutant, and the sexual progeny 

 analyzed. Also, diploids were constructed from 

 all pairwise combinations of the eleven cy mu- 

 tants. The results clearly indicated that six loci 

 were involved. 



In addition to the above strains, two mutants 

 were found having a mutation of a cy gene and a 

 "loss" of the cytoplasmic factor which controls 

 the formation of cytochromes a and b. 



The cytochrome c from some of the cy mutants 

 has been purified and compared to wild type 

 cytochrome c. These mutants have approximately 

 20 per cent of the normal amounts of cytochrome 

 c, but little or no change in respiration. Electro- 

 phoresis and chromatography studies of the 

 cytochromes c are now in progress. 



1. Slonimski and Sherman, in preparation. 



4.50. Nature of a Suppressor Mutation affecting 

 Adenylosuccinase. Joseph S. Gots and Edith 

 G. Gollub (Philadelphia, U.S.A.). 



Mutations occurring in the ade-B locus of 

 Salmonella typhimurium lead to the loss of 

 activity of adenylosuccinase, a bifunctional de- 

 succinylating enzyme required for two sequential 

 reactions in the biosynthesis of adenylic acid. A 

 revertant of one of these mutants was found to 

 be, by phage-mediated transduction, the result 

 of a nonallelic suppressor mutation. We previ- 



ously reported that the adenylosuccinase activity 

 of the suppressor mutant differed from that of 

 the wild type by a number of physicochemical 

 properties and that the differences indicated that 

 the ade-B mutation created an altered protein 

 whose enzymic inactivity could be partially 

 activated by the suppressor mutation. More 

 recent findings make this explanation unlikely. 

 Seven new ade-B mutants have been isolated and 

 identified by recombination analysis as distinctly 

 different single-site alleles of the ade-B locus. 

 These fall into four complementation groups as 

 determined by abortive transduction. Intro- 

 duction of the suppressor gene into any of these 

 mutants leads to a suppressor phenotype. This 

 suggests that the suppressor mutation allows an 

 unrelated enzyme to take over the function of 

 adenylosuccinase. Support for this thesis was 

 obtained by showing distinct immunological 

 specificities between the wild type and suppressor 

 enzymes serving as antigens. The suppressor en- 

 zyme is apparently not related to other known 

 desuccinylating enzymes (e.g. argininosuccinase). 

 Attempts to identify the normal function of the 

 wild type allele of the suppressor locus are being 

 made by searching for (a) natural repressors of 

 the suppressor enzyme and (b) secondary mu- 

 tations in the suppressor locus which could create 

 a complete loss of function. 



4.51. Linkage Groups of Genes controlling Isoleucine, 

 Valine, and Leucine Biosynthesis in Bacillus 

 subtilis. C. Anagnostopoulos and M. Barat 

 (Gif-sur-Yvette, France). 



Genetic analysis was carried out by transfor- 

 mation on a certain number of auxotrophic mu- 

 tants of B. subtilis obtained by u.v. irradiation 

 and requiring either isoleucine (//"), both iso- 

 leucine and valine (il-val-) or leucine (leir). The 

 i'/~ strains were shown to bear mutations on the 

 structural gene for the enzyme threonine deami- 

 nase. The il-val markers are the result of single- 

 step mutations affecting the genes of the enzymes 

 common to both isoleucine and valine pathways. 

 The leir mutants are blocked in the last steps of 

 leucine biosynthesis. 



All /'/ markers were found to be linked to each 

 other but unlinked to the il-val or leu markers. 

 The il-val and leu markers on the other hand are 

 linked and can all be carried on the same piece 

 of transforming DNA as shown by reciprocal 

 crosses and experiments involving double mutants 



It seems therefore that in B. subtilis the bio- 

 synthesis of isoleucine valine and leucine follows 

 the same pathways as determined in other micro- 

 organisms. Some of the genes controlling the last 



53 



