Summary: Using -methyl-'-nitro--nitrosoguanidine, ultraviolet irradiation, ethyl methanesulphonate or 4-nitroquinoline-1-oxide mutagenesis and an enrichment method for the isolation of auxotrophs, 25 mutants with defects in the locus were obtained after screening 41 376 colonies. One of these, , did not complement with any of the other mutants, had a very high reversion rate and had some other properties which usually characterize strains carrying nonsense mutations. All revertants of carried dominant suppressor mutations. A group of suppressors was tested for allele and locus specificity. They were found to suppress only some alleles, and at the same time, some mutations in the and loci. It is proposed that the allele specific and locus non-specific adenine suppressors are suppressors of nonsense mutations.


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