The Arabidopsis thaliana FPS1 gene generates a novel mRNA that encodes a mitochondrial farnesyl-diphosphate synthase isoform.

The enzyme farnesyl-diphosphate synthase (FPS; EC 2.5.1.1./EC 2.5.1. 10) catalyzes the synthesis of farnesyl diphosphate from isopentenyl diphosphate and dimethylallyl diphosphate. FPS is considered to play a key role in isoprenoid biosynthesis. We have reported previously that Arabidopsis thaliana contains two differentially expressed genes, FPS1 and FPS2, encoding two highly similar FPS isoforms, FPS1 and FPS2, (Cunillera, N., Arró, M., Delourme, D., Karst, F., Boronat, A., and Ferrer, A. (1996) J. Biol. Chem. 271, 7774-7780). In this paper we report the characterization of a novel Arabidopsis FPS mRNA (FPS1L mRNA) derived from the FPS1 gene. A cDNA corresponding to the FPS1L mRNA was cloned using a reverse transcription-polymerase chain reaction strategy. Northern blot analysis showed that the two FPS1-derived mRNAs are differentially expressed. The FPS1L mRNA accumulates preferentially in inflorescences, whereas the previously reported FPS1 mRNA (FPS1S mRNA) is predominantly expressed in roots and inflorescences. FPS1L mRNA contains an in-frame AUG start codon located 123 nucleotides upstream of the AUG codon used in the translation of the FPS1S isoform. Translation of the FPS1L mRNA from the upstream AUG codon generates a novel FPS1 isoform (FPS1L) with an NH2-terminal extension of 41 amino acid residues, which has all the characteristics of a mitochondrial transit peptide. The functionality of the FPS1L NH2-terminal extension as a mitochondrial transit peptide was demonstrated by its ability to direct a passenger protein to yeast mitochondria in vivo and by in vitro import experiments using purified plant mitochondria. The Arabidopsis FPS1L isoform is the first FPS reported to contain a mitochondrial transit peptide.