SNQ1 gene function is required for the expression of resistance to 4NQO in wild-type yeast. The sequence of a 3.7 kb yeast DNA containing the gene SNQ1 was determined. The SNQ1 gene consists of an open reading frame of 1641 bp and encodes, according to the hydrophobicity analysis of the putative protein, a transmembrane protein of 547 amino acids. Homology searches in yeast genome databanks revealed a 100% sequence homology with gene ATR1 which controls resistance to aminotriazole in S. cerevisiae. Pre-treatment of wild-type yeast, but not of snq1-0::LEU2 disruption mutants, with sublethal doses of aminotriazole induced hyper-resistance to 4-nitroquinoline-N-oxide. Partial deletion of the nucleotide sequence coding for a putative ATP-binding site has no, or little, influence on resistance to 4NQO whereas total deletion of the region coding for this ATP-binding domain leads to 4NQO-sensitive null-mutants.

The hyperresistance to 4-nitroquinoline-N-oxide (4-NQO) and formaldehyde (FA) of yeast strains transformed with the multi-copy plasmids pAR172 and pAR184, respectively, is due to the two genes, SNQ and SFA, which are present on these plasmids. Restriction analysis revealed the maximal size of SFA as 2.7 kb and of SNQ as 2.2 kb, including transcription control elements. The presence of the smallest 2.7 kb subclone carrying SFA increased hyperresistance to formaldehyde fivefold over that of the original pAR184 isolate. No such increase in hyperresistance to 4-NQO was seen with the smaller subclones of the pAR172 isolate. Disruption of the SFA gene led to a threefold increase in sensitivity to FA as compared with the wild type. Expression of gene SNQ introduced on a multi-copy vector into haploid yeast mutants rad2, rad3, and snm1 did not complement these mutations that block excision repair.

The genes SNQ and SFA confer hyperresistance to 4-NQO and FA when present on a multi-copy plasmid in yeast. Both are non-essential genes since transplacement of SNQ by a disrupted snq-0::LEU2 yielded stable and viable haploid integrants. Southern analysis revealed that SNQ and SFA are single-loci genes, and OFAGE analysis showed that they are located on chromosome XIII and IV, respectively. Northern blot analysis of SNQ and SFA revealed poly(A)+ RNA transcripts of 2 kb and 1.7 kb, respectively. Nuclease S1 mapping showed SNQ to have a coding region of 1.6 kb and SFA, one of 1.3 kb. The 5' coding regions were determined for both genes, while the 3' end could only be determined for gene SNQ. Both genes do not appear to contain introns. The SFA locus was also mapped by transposon mutagenesis. Tn10-LUK integrants disrupted the SFA gene function at sites that were determined by subcloning to lie within the SFA transcription unit.