PEROXISOMAL AND MITOCHONDRIAL CARNITINE ACETYLTRANSFERASES OF SACCHAROMYCES-CEREVISIAE ARE ENCODED BY A SINGLE-GENE

Carnitine acetyltransferase (CAT) is present in mitochondria and perox isomes of oleate-grown Saccharomyces cerevisiae. Both proteins are enc oded by the same gene, YCAT, which encodes a protein with a mitochondr ial targeting signal (MTS) at the N-terminus, and a peroxisomal target ing signal type 1 (PTS-1) at the C-terminus. Deletion of both motifs r evealed the presence of an additional internal targeting sequence. Imp ort of CAT via this internal signal was shown to be dependent on PAS10 , a protein which is required for the import of PTS-1 containing prote ins, An interaction of PAS10 with this internal targeting signal was d emonstrated using the yeast two-hybrid technique. Expression of the YC AT gene behind a heterologous promoter resulted in loss of peroxisomal targeting, indicating that differential targeting is controlled at tr anscriptional or translational level. Determination of the 5'-ends of YCAT mRNAs revealed that YCAT transcripts initiating after the first A UG were present in oleate-grown cells. These transcripts were virtuall y absent in acetate- or glycerol-grown cells, We propose that in respo nse to oleate, shorter transcripts are produced from which the peroxis omal form of CAT is translated, resulting in a CAT protein without a M TS, which can be targeted to peroxisomes.