1s Restricted to Meristematic and Floral Tissues

The synthesis of mevalonate, which is considered the first rate-limiting step in isoprenoid biosynthesis, is catalyzed by the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR; EC 1.1.1.34). In Arabidopsis, HMGR is encoded by two differentially expressed genes (HMG1 and HMGP). The transcriptional activity of the HMGP gene was studied after fusing different regions of its 5' flanking region to the P-glucuronidase (GUS) reporter gene and transforming the resulting constructs into tobacco plants. The spatial and temporal expression directed by the HMGP promoter in the transgenic plants is consistent with the expression pattern previously established by RNA analysis using an HMGBspecific probe. HMGP expression is restricted to meristematic (root t ip and shoot apex) and floral (secretory zone of the stigma, mature pollen grains, gynoecium vascular tissue, and fertilized ovules) tissues. Deletion analysis of the HMGP 5'flanking region was conducted in transgenic plants and transfected protoplasts. The region containing nucleotides -857 to +64 of the HMGP gene was sufficient to confer high levels of expression in both floral and meristematic tissues, although deletion to nucleotide -503 resulted in almost complete loss of expression. Sequences contained within the 5' transcribed, untranslated region are also important for gene expression. The biological significance of the restricted pattern of expression of HMGP is also discussed.