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The yeast COX5a and COX5b genes encode isoforms of subunit Va of the mitochondrial inner membrane protein complex cytochrome c oxidase. These genes have been shown to be inversely regulated at the level of transcription by oxygen, which functions through the metabolic coeffector heme. In earlier studies we identified several regulatory elements that control transcriptional activation and aerobic repression of one of these genes, COX5b. Here, we report the isolation of trans-acting mutants that are defective in the aerobic repression of COX5b transcription. The mutants fall into two complementation groups. One group specifies ROX1, which encodes a product reported to be involved in transcriptional repression. The other group identified the gene we have designated ORD1. Mutations in ORD1 cause overexpression of COX5b aerobically but do not affect the expression of the hypoxic genes CYC7, HEM13, and ANB1. ORD1 mutations also do not affect the expression of the aerobic genes COX5a, CYC1, ROX1, ROX3, and TIF51A. The yeast genome contains a single ORD1 gene that resides on chromosome XI. Strains carrying chromosomal deletions of the ORD1 locus are viable and exhibit phenotypes similar to, but less severe than, that of the original mutant. The nucleotide sequence of ORD1 revealed that it is identical to IXR1, a yeast gene whose product contains two high mobility group boxes, binds to platinated DNA, and confers sensitivity to the antitumor drug cisplatin. Consistent with the latter observations, we found that the ORD1 product could bind to both the upstream region of COX5b and to DNA modified with cisplatin.

proceedings of the national academy of sciences of the united states of america

The ORD1 gene encodes a transcription factor involved in oxygen regulation and is identical to IXR1, a gene that confers cisplatin sensitivity to Saccharomyces cerevisiae.