Plastoquinones play important roles as electron carriers in the light-dependent reactions of photosynthesis and also as a cofactor of phytoene desaturation in the synthesis of carotenoid. A plastoquinone-9 (PQ-9) was identified in Frey-Wyssling organelles of fresh rubber latex from Hevea brasiliensis. This indicates that a Hevea solanesyl diphosphate synthase (HbSDS) must be present for the synthesis of the C45 prenyl side chain of the PQ-9 found in the rubber latex. Based on the sequence information of other trans-prenyl diphosphate synthases, the cDNA encoding HbSDS was cloned from a Hevea rubber latex cDNA library. Sequence analysis revealed the presence of an ORF consisting of 1254 bp capable of encoding a 46,095 Da polypeptide. The deduced amino acid sequence of the clone contained all conserved regions found in typical trans-prenyl chain elongating enzymes. This cDNA, expressed in Escherichia coli cells as an insoluble but Trx-His-tagged fusion protein, showed a distinct solanesyl diphosphate synthase activity in the presence of Triton X-100. The expression level of HbSDS mRNA in latex was found to be higher than those in leaves, stems and roots. The expressed HbSDS is suggested to be a specific enzyme in the Frey-Wyssling particles of rubber latex.
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