Oxygen regulation of anaerobic and aerobic genes mediated by a common factor in yeast

The expression of a number of yeast genes is regulated by oxygen levels. While many of these are known to be induced in the presence of oxygen, we have described a gene, ANB1, that responds in the opposite fashion, being expressed only under anaerobic conditions. To identify genes involved in regulation of ANB1 and other oxygen-regulated genes, we selected mutations causing constitutive expression of ANB1, using a fusion of the ANB1 modulator segment to the CYC1 gene. A number of trans-acting mutations affecting a gene designated ROX1 caused constitutive expression of both the fused and wild-type genes, indicating that the ROX1 gene product operates through the ANB1 modulator sequence at the level of transcription. The mutant alleles of ROX1 fall into two phenotypic classes. The rox1-a class is semi-dominant, and the rox1-b class is recessive. One mutant, rox1-a1, is pleiotropic and causes constitutive expression of three oxygen-induced genes--CYC1, SOD (superoxide dismutase), and tr-1 (an oxygen-induced gene with homology to ANB1)--as well as constitutive expression of the oxygen-repressed ANB1 gene. Alleles of the rox1-b class cause constitutive expression of ANB1 but do not affect expression of the oxygen-induced genes tested. The pleiotropy of the rox1-a1 mutant indicates that the ROX1 gene product is involved in coordinate expression of both oxygen-induced and oxygen-repressed genes.