Both the REO1 and ROX1 genes are thought to encode heme-dependent, transcriptional repressors of hypoxic genes in Saccharomyces cerevisiae. However, genetic complementation studies have yielded conflicting results about whether these are the same or different genes. Because of the central importance of these repressors, which control the expression of nearly all known hypoxic genes in yeast, we have sought to resolve this confusion by comparing the phenotypes of reo1 and rox1 mutants using Northern-blot analyses, performing additional complementation studies, and sequencing the ROX1 gene in reel strains. Northern-blot analyses of a reel strain show wild-type expression of the aerobic genes examined, but de-repression of the Rox1-regulated, hypoxic genes. Aerobic transcript levels of these hypoxic genes were also de-repressed in a diploid strain created by mating a rox1 disrupted strain with a reel strain, indicating that genetic complementation did not occur between these two strains. Sequence analyses of ROX1 in reel strains reveal a frameshift mutation in the 5'-end of its coding region, resulting in a nonsense codon in the sixth position. Taken together, these results provide compelling evidence that reel is an allele of ROX1.
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