Polyprenyldiphosphate: 4-hydroxybenzoate polyprenyltransferase (4-HB polyprenyltransferase) is a key enzyme in ubiquinone biosynthesis in E. coli, encoded by the gene ubiA. By overexpression of ubiA and isolation of the membrane fraction, the enzyme was enriched approx. 3000-fold and characterized. The enzyme is membrane-bound and could not be solubilized by hypotonic buffer or detergent treatment. The enzymatic activity is optimal at pH 7.8 and depends on the presence of magnesium ions. Geranyldiphosphate (GPP), all-trans-farnesyldiphosphate (FPP) and all-trans-solanesyldiphosphate (SPP) are accepted as side chain precursors. The apparent Km values for these substances are are 254 microM, 22 microM and 31 microM, respectively. No reaction was observed with omega-t2-c5-octaprenyldiphosphate, in which five double bounds have cis-configuration. The reaction is stimulated by 0.01% CHAPS, but strongly inhibited by sodiumdeoxycholate, Tween 80 and Triton X-100. The amino acid sequence shows striking similarities to 4-HB hexaprenyltransferase from yeast. Sequence homologies to other prenyltransferases are discussed.