THE SACCHAROMYCES-CEREVISIAE RTG2 GENE IS A REGULATOR OF ACONITASE EXPRESSION UNDER CATABOLITE REPRESSION CONDITIONS

The ACO1 gene, encoding mitochondrial aconitase of Saccharomyces cerev isiae, is required both for oxidative metabolism and for glutamate pro totrophy. This gene is subject to catabolite repression; the ACO1 mRNX level is further reduced when glutamate is supplied with glucose. To further explore regulation of ACO1 expression, we have screened for mu tations that reduce expression of an ACO1-lacZ fusion borne on a multi copy vector. We identified a gene required for wild-type expression of ACO1 only under catabolite repression conditions. Sequencing of the c orresponding cloned gene revealed that it is identical to RTG2 previou sly cloned as a pivotal gene in controlling interorganelle retrograde communication. Cells containing either the original rtg2-2 mutation or a null rtg2 allele are not petite but show a residual growth on minim um glucose medium with ammonium sulfate as the sole nitrogen source. T his growth defect is partially restored by supplying aspartate or thre onine, and fully with glutamate or proline supplement. Surprisingly, t his phenotype is not observed on complete medium lacking either of the se amino acids. In addition, a genetic analysis revealed an interactio n between RTG2 and ASP5 (encoding aspartate amino transferase), thus s upporting our hypothesis that RTG2 may be involved in the control of s everal anaplerotic pathways.