THE CIT3 GENE OF SACCHAROMYCES-CEREVISIAE ENCODES A 2ND-MITOCHONDRIALISOFORM OF CITRATE SYNTHASE

We have identified a third citrate synthase gene in Saccharomyces cere visiae which we have called CIT3. Complementation of a citrate synthas e-deficient strain of Escherichia coli by lacZ::CIT3 gene fusions demo nstrated that the CIT3 gene encodes an active citrate synthase. The CI T3 gene seems to be regulated in the same way as CIT1, which encodes t he mitochondrial isoform of citrate synthase. Deletion of the CIT3 gen e in a Delta cit1 background severely reduced growth on the respirator y substrate glycerol, whilst multiple copies of the CIT3 gene in a Del ta cit1 background significantly improved growth on acetate. In vitro import experiments showed that cit3p is transported into the mitochond ria. Taken together, these data show that the CIT3 gene encodes a seco nd mitochondrial isoform of citrate synthase.