EXPRESSION OF THE ARABIDOPSIS HMG2 GENE, ENCODING 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE, IS RESTRICTED TO MERISTEMATIC AND FLORALTISSUES

The synthesis of mevalonate, which is considered the first rate-limiti ng step in isoprenoid biosynthesis, is catalyzed by the enzyme 3-hydro xy-3-methylglutaryl coenzyme A reductase (HMGR; EC 1.1.1.34). In Arabi dopsis, HMGR is encoded by two differentially expressed genes (HMG1 an d HMG2). The transcriptional activity of the HMG2 gene was studied aft er fusing different regions of its 5' flanking region to the beta-gluc uronidase (GUS) reporter gene and transforming the resulting construct s into tobacco plants. The spatial and temporal expression directed by the HMG2 promoter in the transgenic plants is consistent with the exp ression pattern previously established by RNA analysis using an HMG2-s pecific probe, HMG2 expression is restricted to meristematic (root tip and shoot apex) and floral (secretory zone of the stigma, mature poll en grains, gynoecium vascular tissue, and fertilized ovules) tissues. Deletion analysis of the HMG2 5' flanking region was conducted in tran sgenic plants and transfected protoplasts. The region containing nucle otides -857 to +64 of the HMG2 gene was sufficient to confer high leve ls of expression in both floral and meristematic tissues, although del etion to nucleotide -503 resulted in almost complete loss of expressio n. Sequences contained within the 5' transcribed, untranslated region are also important for gene expression. The biological significance of the restricted pattern of expression of HMG2 is also discussed.