Regulation of the am-1 locus in Neurospora: evidence of independent control of allerlic recombination and gene expression.

ENES affecting the frequency of recombination between allelic differences have been found in Neurospora crassa (JESSOP and D. G. CATCHESIDE 1965; D. G. CATCHESIDE 1966; JHA 1967). In the two most extensively studied cases, crosses homozygous for rec (a general symbol for genes affecting recombination) exhibit allelic recombination frequencies an order of magnitude or more greater than those containing the recf allele. The rec genes are specific, rec-2 affecting recombination frequency between his2 (histidine-2) alleles and rec-3 that between am-l (amination-l) alleles. There is no interaction between rec-2 and am-1 nor between rec-3 and his-2 (D. G. CATCHESIDE 1966). Neither rec gene is contiguous with the structural gene which it affects. Indeed, rec-3 is located about 12 map units proximal to mating type in linkage group I whilst am-1 is in linkage group V and rec-2 is about 19 units distal to his-2 also in linkage group V (D. G. CATCHESIDE 1966 and unpublished observations). The dominant nature, specificity and remoteness of the recf genes from their points of action has led to the proposition that they are regulatory genes (D. G. CATCHESIDE 1966; WHtTEHousE 1966) analogous to those described in other organisms (MCCLINTOCK 1957; JACOB and MONOD 1961). Regulation by rec+ is most simply envisaged as acting directly at the gene level by the agency of a gene product which prevents either the formation of hybrid DNA or the correction of any consequently mispaired bases. The polarized effect of rec on either or both of these processes (D. G. CATCHESIDE 1967) is most simply described in terms of a direct control at the gene level (D. E. A. CATCHESIDE 1967). This paper considers the possibility that the effects of rec genes on recombination frequency may reflect modifications of an integrated, dual purpose, control system acting at the gene level; D. G. CATCHESIDE (1966) and WHITEHOUSE (1966) have suggested lbat control of recombination and transcription may be integrated. The rec-3, an-2 pair has been chosen for investigation as it has a number of convenient properties. Mutants at the am-1 locus are known to produce nicotine adenine dinucleotide phosphate-specific glutamate dehydrogenase (NADP-GDH; Enzyme Commission number 1.4.1.4) varieties which differ in properties from the wild-type enzyme (FINCHAM 1962:; FINCHAM and STADLER 1965). Hence, the am-2 locus