Catalyzes the decarboxylation of 3-octaprenyl-4-hydroxy benzoate to 2-octaprenylphenol, an intermediate step in ubiquinone biosynthesis. a 4-hydroxy-3-all-trans-polyprenylbenzoate + H(+) = a 2-all-trans-polyprenylphenol + CO2 Binds 1 prenylated FMN (prenyl-FMN) per subunit. Cofactor biosynthesis; ubiquinone biosynthesis. Homohexamer. Belongs to the UbiD family.