A Histidine-3 Mutant, in Neurospora crassa, due to an Interchange

Abstract

The his-3 mutant TM429, which has an absolute requirement for histidine, appears to be due to a break in the his-3+ gene just distal to the site of K874. In consequence the proximal three-quarters of the his-3 gene is separated from the distal quarter and from the cog locus, between his-3 and ad-3, at or near which most recombination in this region is initiated. The original stock of TM429 carried cog+. Allelic recombination between TM429 and his-3 genes with sites of difference proximal to TM429 is unaffected by the substitution of rec-2 for rec-2+, unless cog+ is also present in the second parent. Allelic recombination between TM429 and his-3 genes with sites of difference distal to TM429 is increased by rec-2, as also is non-allelic recombination between TM429 and ad-3, irrespective of whether cog+ or cog is in the second parent. The data are explicable if cog+ is very strongly competitive as the site for initiating recombination and if a recombination event running towards his-3 is not also established at once in the cog chromosome.