Genes conferring resistance to 8-aza adenine in Neurospora crassa and the variability of resistant alleles in the aza-l locus with respect to excretion of purines

Summary

Strains inhibited by analogues of aromatic amino acids due to mutations in the 5mt locus were also sensitive to the purine analogues, 8-aza adenine, 8-aza guanine and 2,6-diaminopurine. Adenine or hypoxanthine and the respective ribosides relieved inhibition due to purine analogues but guanine or guanosine were not effective. Using resistance to 8-aza adenine and sensitivity to DL-4-methyl tryptophan as criteria aza-1 ^r and aza-2 ^r mutants were isolated and both loci were located in linkage group I the former was 10–20 units distal to the mating type and the latter approximately 2 units in the vicinity of mating type. The four allelic aza-1 ^r strains were resistant to 8-aza adenine and 2,6-diaminopurine but one of them showed increased resistance to 8-aza guanine also. The aza-2 ^r strain was resistant to both 8-aza analogues but was comparable to the parental aza-2 ^s strain in its sensitivity to 2,6-diaminopurine. One aza-1 ^r was distinguished by its ability to excrete hypoxanthine and/or inosine and this ability always segregated with the aza-1 ^r allele and appears to be a consequence of a single mutation in the aza-1 locus. In heterokaryon one aza-1 ^r and one aza-2 ^r alleles were found to be recessive to the respective wild type alleles. Uptake of exogenous adenine was reduced in germinating conidia of one aza-1 ^r and one aza-2 ^r strain when compared to a parental aza-1 ^s, aza-2^s strain but the low affinity of 8-aza adenine to compete with adenine even in the sensitive strain indicated a need for direct study of the transport and metabolic reactions of the analogue. If an altered purine phosphoribosyl transferase is involved in the aza-1 ^r phenotype, an analogy with the Lesch-Nyhan syndrome of man, with respect to analogue-resistance and altered regulation of purine metabolism, seems to be appropriate.