Cysteine Mutant Strains of Neurospora'

UTANT strains of Neurospora crassa that are unable to utilize inorganic sulfur, but which are capable of growth on minimal medium supplemented with either cysteine or methionine, are obtained readily. It is surprising, therefore, that little genetic information has been available for cysteine mutants of Neurospora, other than those located in linkage group VI (STADLER 1956a,b, 1959a,b; STADLER and TOWE 1963). Recent progress in the elucidation of cysteine synthesis in Salmmella typhimurium ( DREYFUSS and MONTY 1963) has stimulated renewed interest in the synthetic pathway in Neurospora (LEINWEBER and MONTY 1964, 1965). It therefore seemed desirable to obtain genetic information on the strains available for biochemical analyses. In an earlier study. complementation tests demonstrated seven physiological classes among 62 cysteine mutants (MURRAY 1960). Two factors have contributed to the paucity of genetic information on these mutants. ( 1 ) Three of these groups of mutants are at loci very far out from the centromere. (2) Cysteine mutant strains apparently acquire additional mutations at other loci. leading to strains that are doubly blocked in cysteine synthesis. This problem has not been investigated systematically, but at least some originally single-mutant stocks kept by the Fungal Genetic Stock Center Collection (FGSC; Dartmouth College, Hanover, N.H.), by DR. D. R. STADLER and MRS. AGNES TOWE (personal communication). and by me, were found to be double mutants. I found a culture of cys-5 (35001 [FGSC No. 4281 ) to be a double mutant (cys-5, cys-2 j , and have isolated a probable cys-5 allele from a culture of cys-2 (71310[FGSC No. 9681). (Both FGSC stocks have since been replaced with bona fide single-mutant strains.) Another cys-2 strain was shown by MRS. AGNES TOWE (personal communication) to have acquired a secondary mutation in, or close to, the cys-5 region. The unambiguous genetic identification of each locus. provided by location on a linkage map, therefore becomes increasingly desirable. All available cysteine mutants have now been classified and mapped. This paper presents a summary of the genetic information on eight (or possibly nine) cysteine loci. Linkage data for six of these loci have not been published previously, although some information has been made available in the Neurospora Newsletter.