Characterization of the yeast HEM2 gene and transcriptional regulation of COX5 and COR1 by heme.

The respiratory deficiency of two noncomplementing mutants of Saccharomyces cerevisiae (C41 and N28) has been shown to be due to mutations in HEM2, the structural gene for delta-aminolevulinate dehydratase. The mutants are unable to convert delta-aminolevulinic acid to porphobilinogen and are not complemented by the hem2 mutant GL4 (Gollub, E. G., Liu, K.-P., Dagan, J., Adlersberg, M., and Sprinson, D. B. (1977) J. Biol. Chem. 252, 2846-2854). A gene capable of complementing the respiratory deficiency of C41 and N28 has been cloned by transformation of a hem2 mutant with a recombinant plasmid library of wild type yeast nuclear DNA. The sequence of the protein encoded by the cloned gene exhibits extensive homology to the recently reported sequence of human delta-aminolevulinate dehydratase (Wetmur, J. G., Bishop, D. F., Cantelmo, C., and Desnick, R. J. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 7703-7707). Several approaches were taken to study the effect of heme on transcription of PET genes known to code for subunit components of respiratory enzymes and of mitochondrial ATPase. The first involved measurements of the steady state levels of mRNAs for subunit 5 of cytochrome oxidase and the beta subunit of F1 ATPase in wild type and in a hem2 mutant. Secondly, transcription of the genes coding for the cytochrome oxidase and ATPase subunits as well as of the COR1 gene coding for the 44-kDa core 1 subunit of coenzyme QH2-cytochrome c reductase was quantitated by fusing the 5'-flanking and part of the coding region of each gene to the lacZ gene of Escherichia coli in vectors capable of integrating into yeast chromosomal DNA. The different lacZ fusions were integrated into nuclear DNA of a wild type strain and of hem2 mutants allowing expression of beta-galactosidase to be studied as a function of intracellular heme. These experiments indicate that the promoters of the genes for subunits of the respiratory complexes are regulated by heme. In contrast, the expression of the ATPase subunit appears to be heme-independent. Because neither subunit 5 of cytochrome oxidase nor the core 1 subunit of coenzyme QH2-cytochrome c reductase are hemoproteins, transcriptional regulation by heme may be a general mechanism for controlling the synthesis of mitochondrial proteins involved in respiration.