The promoter sequences of lettuce cis-prenyltransferase and its binding protein specify gene expression in laticifers.

Promoters of lettuce cis-prenyltransferase 3 (LsCPT3) and CPT-binding protein 2 (LsCBP2) specify gene expression in laticifers, as supported by in situ β-glucuronidase stains and microsection analysis. Lettuce (Lactuca sativa) has articulated laticifers alongside vascular bundles. In the cytoplasm of laticifers, natural rubber (cis-1,4-polyisoprene) is synthesized by cis-prenyltransferase (LsCPT3) and CPT-binding protein (LsCBP2), both of which form an enzyme complex. Here we determined the gene structures of LsCPT3 and LsCBP2 and characterized their promoter activities using β-glucuronidase (GUS) reporter assays in stable transgenic lines of lettuce. LsCPT3 has a single 7.4-kb intron while LsCBP2 has seven introns including a 940-bp intron in the 5'-untranslated region (UTR). Serially truncated LsCPT3 promoters (2.3 kb, 1.6 kb, and 1.1 kb) and the LsCBP2 promoter with (1.7 kb) or without (0.8 kb) the 940-bp introns were fused to GUS to examine their promoter activities. In situ GUS stains of the transgenic plants revealed that the 1.1-kb LsCPT3 and 0.8-kb LsCBP2 promoter without the 5'-UTR intron are sufficient to express GUS exclusively in laticifers. Fluorometric assays showed that the LsCBP2 promoter was several-fold stronger than the CaMV35S promoter and was ~ 400 times stronger than the LsCPT3 promoter in latex. Histochemical analyses confirmed that both promoters express GUS exclusively in laticifers, recognized by characteristic fused multicellular structures. We concluded that both the LsCPT3 and LsCBP2 promoters specify gene expression in laticifers, and the LsCBP2 promoter displays stronger expression than the CaMV35S promoter in laticifers. For the LsCPT3 promoter, it appears that unknown cis-elements outside of the currently examined LsCPT3 promoter are required to enhance LsCPT3 expression.