A thermophilic cyanobacterium Synechococcus elongatus has three different Class I prenyltransferase genes

Prenyltransferases (prenyl diphosphate synthases), which are a broad group of enzymes that catalyze the consecutive condensation of homoallylic diphosphate of isopentenyl diphosphates (IPP, C₅) with allylic diphosphates to synthesize prenyl diphosphates of various chain lengths, have highly conserved regions in their amino acid sequences. Based on the above information, three prenyltransferase homologue genes were cloned from a thermophilic cyanobacterium, Synechococcus elongatus. Through analyses of the reaction products of the enzymes encoded by these genes, it was revealed that one encodes a thermolabile geranylgeranyl (C₂₀) diphosphate synthase, another encodes a farnesyl (C₁₅) diphosphate synthase whose optimal reaction temperature is 60 °C, and the third one encodes a prenyltransferase whose optimal reaction temperature is 75 °C. The last enzyme could catalyze the synthesis of five prenyl diphosphates of farnesyl, geranylgeranyl, geranylfarnesyl (C₂₅), hexaprenyl (C₃₀), and heptaprenyl (C₃₅) diphosphates from dimethylallyl (C₅) diphosphate, geranyl (C₂₀) diphosphate, or farnesyl diphosphate as the allylic substrates. The product specificity of this novel kind of enzyme varied according to the ratio of the allylic and homoallylic substrates. The situations of these three S. elongatus enzymes in a phylogenetic tree of prenyltransferases are discussed in comparison with a mesophilic cyanobacterium of Synechocystis PCC6803, whose complete genome has been reported by Kaneko et al. (1996).