Genetic control of allelic recombination at the histidine-3 locus of Neurospora crassa.

T H E histidine-3 (his-3) locus of Neurospora crassa has aroused considerable interest because it 'controls not only the last step but also at least two early reactions in the biosynthesis of histidine (AHMED, CASE and GILES 1964; CATCHESIDE 1960, 1965). Complementation and genetic analyses of his-3 mutants, showing that noncomplemeiiting mutants, lacking all the three functions, appeared to be localized towards one end of the genetic map. led AHMED, CASE and GILES (1964) to suggest that the his-3 locus constitutes an operon, a polarized unit of coordinated transcription, according to the concept of JACOB and MONOD (1961). CATCHESIDE (1965) pointed out that the complementation map was not uniquely ordered and the data upon which the genetic map is based are subject to genetic and other variation. His tentative genetic map of his-3 locus showed two noncomplementing mutants located at sites distant from other such mutants. It is obvious that the fine structure of this locus needs to be established with certainty and the present study is a step in that direction. JESSOP and CATCHESIDE (1965) and CATCHESIDE (1966a) have shown that specific recombination (rec) genes, rec-l and rec-3, control the frequencies of allelic recombination at the histidine-l and amination loci respectively. They have shown that two wild types, which are often assumed to be fairly isogenic, differ in their rec constitutions. Prototroph frequency is an important criterion for mapping of alleles and in the absence of knowledge about genetic control of this phenomenon our views about fine structure of a gene must remain uncertain. The value of rec genes in the study of phenomena concerned with recombination is obvious. This article reports a genetic factor which affects the recombination frequencies in crosses between his-3 genes. It resembles rec-1 and rec-3, except that its effect is relatively small. Whether it is specific to his-3 is unknown.