The Promoter for Tomato 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Gene 2 Has Unusual Regulatory Elements That Direct High-Level Expression

The promoter region of tomato (Lycopersicon esculentum) 3-hydroxy-3-methylglutaryl coenzyme A reductase gene 2 (HMG2) has been analyzed using the transient expression of HMG2-luciferase fusions in red fruit pericarp. The mRNA for HMG2 accumulates to high levels during fruit ripening, in a pattern that coincides with the synthesis of the carotenoid lycopene. Unlike most promoters, the region that is upstream of the HMG2 TATA element is not required for high-level expression. The 180-bp region containing the TATA element, the 5[prime] untranslated region, and the translation start site are comparable in strength to the full-length 35S cauliflower mosaic virus promoter. Pyrimidine-rich sequences present in the 5[prime] untranslated leader are important in regulating expression. Also, the ATG start region has been found to increase translation efficiency by a factor of 4 to 10. An alternative hairpin secondary structure has been identified surrounding the HMG2 initiator ATG, which could participate in the translational regulation of this locus. HMG2 appears to be a novel class of strong plant promoters that incorporate unusual, positive regulators of gene expression.