The use of an alternative promoter in the Arabidopsis thaliana HMG1 gene generates an mRNA 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 the great variety of isoprenoid compounds and derivatives synthesized in higher plants. It has previously been reported that Arabidopsis thaliana contains two differentially expressed genes, HMG1 and HMG2, that encode two HMGR isoforms (HMGR1 and HMGR2, respectively). This paper reports the characterization of a novel HMGR mRNA (HMGR1L mRNA) derived from the HMG1 gene. This mRNA is initiated 121 bp upstream from the transcription start site previously characterized. In contrast with the previously reported HMGR1 mRNA (HMGR1S mRNA), which is detected at high levels in all tissues of the plant, HMGR1L mRNA is present at relatively low levels and its expression is restricted mostly to seedlings, roots and inflorescences. HMGR1L and HMGR1S mRNAs are transcribed from alternative promoters. HMGR1L mRNA contains an in-phase AUG start codon which allows the synthesis of a novel HMGR isoform (HMGR1L) having 50 additional amino acid residues at its N-terminal end. Using an in vitro transcription-translation system we have shown that HMGR1L is inserted into ER-derived microsomes. It is thus unlikely that the extended N-terminal region of HMGR1L might have a role in targeting the enzyme to plastids or mitochondria. These results support the previous proposal that the endoplasmic reticulum is the only cell compartment for the primary targeting of HMGR in Arabidopsis and reinforce the view that plant HMGR is under the control of complex mechanisms operating at both transcriptional and post-transcriptional levels.