Molecular cloning and mutational analysis of the ddsA gene encoding decaprenyl diphosphate synthase from Gluconobacter suboxydans

Decaprenyl diphosphate (decaprenyl‐ PP ) synthase catalyzes the consecutive condensation of isopentenyl diphosphate with allylic diphosphates to produce decaprenyl‐ PP , which is used for the side chain of ubiquinone (Q)‐10. We have cloned the synthase gene, designated ddsA , from Gluconobacter suboxydans and expressed it in Escherichia coli . Sequence analysis revealed the presence of an ORF of 948 bp capable of encoding a 33 898‐Da polypeptide that displays high similarity (30−50 %) to other prenyl diphosphate synthases. Expression of the ddsA gene complemented the lethality resulting from a defect in the octaprenyl diphosphate synthase gene of E. coli and produced Q‐10, indicating that Q‐10 can substitute for the function of Q‐8. The His‐tagged DdsA protein was purified to characterize its enzymatic properties. This enzyme required detergent (0.05 % Triton X‐100) and 10 mM Mg 2+ , for full activity. The Michaelis constants for geranyl diphosphate, all‐E ‐farnesyl diphosphate and all‐E ‐geranylgeranyl diphosphate were 7.00, 0.50 and 0.32 μM, respectively. Nine single‐amino‐acid substitutions were introduced upstream of conserved region II or VI. Most of the mutants showed a considerable decrease in catalytic activity or shortening of the ultimate chain length. However, the A70G mutant produced a longer‐chain‐length product than wild‐type decaprenyl‐ PP synthase, and the A70Y mutant completely abolished the decaprenyl‐ PP synthase function, indicating that Ala70 is important for enzyme activity and the determination of the chain‐length properties of DdsA.