Nuclear control of the messenger RNA expression for mitochondrial ATPase subunit 9 in a new yeast mutant.

A new temperature-sensitive nuclear mutant of Saccharomyces cerevisiae altered in the expression of the mitochondrially encoded ATPase 9 gene was identified. Analysis of several mitochondrial transcripts reveals the selective loss of the messenger RNA for the ATPase 9 subunit under restrictive temperature conditions. This defect results in a respiratory deficiency of the yeast cells. RNA synthesis studied with isolated mitochondria gives first evidence that the mitochondrial transcription process does not react in a temperature-sensitive way. Therefore, it is concluded that the loss of the transcript is due to specific posttranscriptional degradation. Further on it is demonstrated that all the observed effects in the mutant are caused by only one mutated nuclear gene. The mutant strain was transformed with a genomic yeast library and a complementary DNA fragment was obtained. Molecular characterization of the respective DNA clone identified a new gene, which we propose to call NCA1 (Nuclear Control of ATPase messenger RNA expression). Complete inactivation of this gene by insertion mutagenesis in the chromosome leads to a permanent respiratory defect of the cell. Localization of the insertion locus in the yeast genome and complementation studies with the temperature-sensitive mutant indicate that the two mutations are allelic.