Section 1 — Complex Loci 



two are in different complementation groups and 

 exhibit different levels of ability to form indole; 

 the third neither complements nor forms indole. 

 The relationships between the NIU mutants 

 will be described in detail and their possible 

 significance in terms of gene structure and func- 

 tion will be discussed. 



Supported by grants from the National 

 Science Foundation. 



1.15. Further Analysis of Interallelic Complementa- 

 tion at the leu-2 Locus of Neurospora crassa. 



S. R. Gross (Durham, U.S.A.). 



A previous analysis of the complementation 

 behavior of a large number of leu-2 mutants of 

 Neurospora crassa yielded a complementation 

 map that was linear, overlapping and conti- 

 nuous/ 1 ) Evidence was presented that indicated 

 that the polypeptide whose structure was deter- 

 mined by the leu-2 gene was at least one of two 

 different polypeptide structural units of an 

 enzyme (3-carboxy-P-hydroxyisocaproic acid 

 isomerase, an enzyme which catalyzes the iso- 

 merization of (3-carboxy-(3-hydroxyisocaproate 

 and cc-hydroxy-(3-carboxyisocaproate, interme- 

 diates in leucine biosynthesis. An analysis of the 

 complementation interaction between leu-2 and 

 leu-3 alleles as well as enzymological analyses 

 indicated that both the leu-2 and leu-3 loci were 

 involved in the determination of the structure of 

 P-carboxy-[3-hydroxyisocaproic acid isomerase. 

 The evidence obtained was interpreted as indicat- 

 ing that complementation between leu-2 mu- 

 tants resulted from a protein-protein interac- 

 tion in the formation of a polymer consisting of 

 a chains coded by the leu-2 gene and p chains 

 coded by the leu-3 gene, and that at least in the 

 case of enzymes synthesized as a result of com- 

 plementation, the isomerase consisted of at 

 least two a and two p polypeptide chains. A 

 recent study of the physical chemistry of the 

 isomerase obtained from several complementing 

 pairs of leu-2 mutants has revealed that the 

 molecular weight of the normal enzyme as well 

 as that of the heteroallelic enzymes are the same, 

 ca. 80,000. The enzymes obtained by complemen- 

 tation are remarkably similar to the normal 

 enzyme with respect to the functional area of the 

 protein. Several physical properties of the 

 heteroallelic enzymes differ markedly from that 

 of the isomerase enzyme. These differences, 

 however, are probably confined to areas not 

 directly involved in substrate binding. 



1.16. Homology Tests on X-ray-induced Recessive 

 Lethal Mutations in the ad-3 Region of 

 Neurospora crassa. F. J. de Serres (Oak Ridge, 

 U.S.A.). 



Forward-mutation experiments on a balanced 

 heterokaryon^ 1 ) have shown that 76 per cent 

 of the ad-3 mutations are recessive lethal (ad-3 RL ) 

 on adenine-supplemented medium and that 40 

 per cent of these were ad-3 A ad-3B RJj double 

 mutants. Atwood and Mukai( 2) have shown that 

 such recessive lethal mutations can be analyzed 

 by homology tests. Such tests on the ad-3 RIj 

 mutations show that they form an inclusive 

 complex; all combinations show homology 

 except some of the ad-3A Rlj +ad-3B RL combina- 

 tions. The apparent homology of certain ad-3A RL 

 + ad-3B RJj combinations is of particular interest 

 since the trikaryon involving two such mutations 

 in combination with a viable double mutant 

 (from a cross of two viable mutants ad-3A v 

 x ad-3B v = ad-3A v ad-3B v ) is capable of 

 growth on minimal medium. The behavior of 

 these trikaryons shows that there is an intact 

 essential region (designated region X) present in 

 the ad-3A v ad-3B v double mutant that is non- 

 functional in any of the ad-3A RJj or ad-3B RL 

 mutations showing homology. The simplest 

 assumption is that region X is located between 

 the ad-3 A and ad-3B loci, and present data are 

 consistent with this hypothesis. The interaction 

 patterns of such ad-3 RL mutations in homology 

 tests offer a unique opportunity for the analysis 

 of the genetic composition of the ad-3 region, 

 and the results of these tests will be described 

 in terms of a complementation map. 



1. Gross, Proc. Nat. Acad. Sci. 48, 922, 1962. 



1. De Serres and Osterbind, Genetics 47, 793- 

 796, 1962. 



2. PN AS 39, 1027-35, 1953. 



1.17. Complementation and Genetic Mapping of pan-2 

 Mutants Induced in a Reversion of a Primary 

 pan-2 Mutant. Mary Case (New Haven, 

 U.S.A.). 



Twenty-nine secondary pan-2 (pantothenic 

 acid-requiring) mutants have been obtained 

 following X-irradiation of a reversion induced 

 by X-rays in a complementing primary pan-2 

 mutant B5. Thirteen of the 29 mutants comple- 

 ment with at least one primary mutant, and all 

 13 can be positioned on a linear complementation 

 map of the locus. The behavior of the secondary 

 mutants establishes the existence of an additional 

 complementation unit at the pan-2 locus. All 



