The isoprenoid chain of ubiquinone (Q) is determined by trans-polyprenyl diphosphate synthase in micro-organisms and presumably in mammals. Because mice and humans produce Q9 and Q10, they are expected to possess solanesyl and decaprenyl diphosphate synthases as the determining enzyme for a type of ubiquinone. Here we show that murine and human solanesyl and decaprenyl diphosphate synthases are heterotetramers composed of newly characterized hDPS1 (mSPS1) and hDLP1 (mDLP1), which have been identified as orthologs of Schizosaccharomyces pombe Dps1 and Dlp1, respectively. Whereas hDPS1 or mSPS1 can complement the S. pombe dps1 disruptant, neither hDLP1 nor mDLP1 could complement the S. pombe dLp1 disruptant. Thus, only hDPS1 and mSPS1 are functional orthologs of SpDps1. Escherichia coli was engineered to express murine and human SpDps1 and/or SpDlp1 homologs and their ubiquinone types were determined. Whereas transformants expressing a single component produced only Q8 of E. coli origin, double transformants expressing mSPS1 and mDLP1 or hDPS1 and hDLP1 produced Q9 or Q10, respectively, and an in vitro activity of solanesyl or decaprenyl diphosphate synthase was verified. The complex size of the human and murine long-chain trans-prenyl diphosphate synthases, as estimated by gel-filtration chromatography, indicates that they consist of heterotetramers. Expression in E. coli of heterologous combinations, namely, mSPS1 and hDLP1 or hDPS1 and mDLP1, generated both Q9 and Q10, indicating both components are involved in determining the ubiquinone side chain. Thus, we identified the components of the enzymes that determine the side chain of ubiquinone in mammals and they resembles the S. pombe, but not plant or Saccharomyces cerevisiae, type of enzyme.