Isolation and structural characterization of a cDNA encoding Arabidopsis thaliana 3-hydroxy-3-methylglutaryl coenzyme A reductase

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Abstract
The enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34) catalyses the synthesis of mevalonate, the specific precursor of all isoprenoid compounds present in plants. We have characterized two overlapping cDNA clones that encompass the entire transcription unit of an HMG-CoA reductase gene from Arabidopsis thaliana. The transcription product has an upstream non-coding sequence of 70 nucleotides preceding an open reading frame of 1776 bases and a 3′ untranslated region in which two alternative polyadenylation sites have been found. The analysis of the nucleotide sequence reveals that the cDNA encodes a polypeptide of 592 residues with a molecular mass of 63 605 Da. The hydropathy profile of the protein indicates the presence of two highly hydrophobic domains near the N-terminus. A sequence of 407 amino acids corresponding to the C-terminal part of the protein (residues 172–579), which presumably contains the catalytic site, shows a high level of similarity to the region containing the catalytic site of the hamster, human, yeast and Drosophila enzymes. The N-terminal domain contains two putative membrane-spanning regions, in contrast to the enzyme from other organisms which has seven trans-membrane regions. A. thaliana contains two different HMG-CoA reductase genes (HMG1 and HMG2), as estimated by gene cloning and Southern blot analysis. Northern blot analysis reveals a single transcript of 2.4 kb in leaves and seedlings, which presumably corresponds to the expression of the HMG1 gene.