NCA2, A SECOND NUCLEAR GENE REQUIRED FOR THE CONTROL OF MITOCHONDRIALSYNTHESIS OF SUBUNIT-6 AND SUBUNIT-8 OF ATP SYNTHASE IN SACCHAROMYCES-CEREVISIAE

Respiratory-competent nuclear mutants have been isolated which present ed a cryosensitive phenotype on a non-fermentable carbon source, due t o a dysfunction of the mitochondrial F1-Fo ATP synthase. This defect r esults from an alteration of the mtDNA-encoded protein synthesis level of subunits 6 and 8 of the Fo sector, due to the simultaneous presenc e of a mutation in two unlinked nuclear genes. These mutations promote a modification of the expression of the cotranscript ATP8-ATP6 (forme rly denoted AAP1-OLI2): this mRNA undergoes a maturation at a unique s ite reaching to two contranscripts of 5.2 and 4.6 kb in length: in the mutant, the relative amount of 5.2 kb cotranscript was greatly lowere d. NCA2 was isolated from a wild-type yeast genomic library by genetic complementation. The relative level of the 5.2 kb transcript, as the synthesis of subunits 6 and 8, was partly restored in the transformed strain. A 1848 nucleotide open reading frame was depicted that encoded an amphiphilic protein of 70,816 Da. Disruption of chromosomal DNA wi thin the reading frame promoted a dramatic decrease of the 5.2 kb mRNA but did not abolish the respiratory competence of a wild-type strain. Hybridization analyses indicated that NCA2 is located on chromosome X VI and produces a single 2750 base transcript.