A central problem in our understanding of mitochondrial (mt) function remains the question of how coordinate transcriptional control is accomplished between nucleus and mitochondria. Here, we report the initial characterization of a protein of previously unknown function, the product of the YMR030 W gene, that appears to mediate such coordinate gene expression. Expression of YMR030 W is glucose-repressible; a deletion mutant for this gene shows a severe growth defect on glycerol-, but not glucose- or ethanol-based medium. In that mutant, transcript levels from GUT1 and GUT2 are highly attenuated compared with those of the wild-type parent when both are grown on glycerol-based medium. Under the same growth conditions, transcripts from the mt OLI1 gene, which has one copy of a mt upstream activating sequence (UAS) in its 5'-flanking region, are attenuated in the DeltaYMR030 W mutant, but mRNA from the mt COX3 ( OXI2) gene, which lacks the mt UAS, are not. Some nuclear genes encoding mt-related proteins also show low transcript levels in the DeltaYMR030 W mutant in comparison with those of the wild-type parent strain during glycerol-based growth. Localization of the protein, via its expression fused to green fluorescent protein, indicates that it is present in both nucleus and mitochondria, supporting a respiration-related transcriptional role for this gene product in both cellular genetic compartments. Because of its role in both respiratory growth and mt function, we designate the YMR030 W coding sequence RSF1 (respiration factor 1).