TH USE OF AN ALTERNATIVE PROMOTER IN THE ARABIDOPSIS-THALIANA HMG1 GENE GENERATES AN MESSENGER-RNA THAT ENCODES A NOVEL 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE ISOFORM WITH AN EXTENDED N-TERMINAL REGION

The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR, EC 1.1.1.34 ) catalyses the synthesis of mevalonate, the committed precursor of th e great variety of isoprenoid compounds and derivatives synthesized in higher plants. It has previously been reported that Arabidopsis thali ana contains two differentially expressed genes, HMG1 and HMG2,that en code two HMGR isoforms (HMGR1 and HMGR2, respectively). This paper rep orts the characterization of a novel HMGR mRNA (HMGR1L mRNA) derived f rom the HMG1 gene. This mRNA is initiated 121 bp upstream from the tra nscription start site previously characterized. In contrast with the p reviously reported HMGR1 mRNA (HMGR1S mRNA), which is detected at high levels in all tissues of the plant, HMGR1L mRNA is present at relativ ely low levels and its expression is restricted mostly to seedlings, r oots and inflorescences. HMGR1L and HMGR1S mRNAs are transcribed from alternative promoters. HMGR1L mRNA contains an in-phase AUG start codo n which allows the synthesis of a novel HMGR isoform (HMGR1L) having 5 0 additional amino acid residues at its hi-terminal end. Using an in v itro transcription-translation system we have shown that HMGR1L is ins erted into ER-derived microsomes. It is thus unlikely that the extende d N-terminal region of HMGR1L might have a role in targeting the enzym e to plastids or mitochondria. These results support the previous prop osal that the endoplasmic reticulum is the only cell compartment for t he primary targeting of HMGR in Arabidopsis and reinforce the view tha t plant HMGR is under the control of complex mechanisms operating at b oth transcriptional and post-transcriptional levels.