O-methyltransferase that catalyzes the 2 O-methylation steps in the ubiquinone biosynthetic pathway. Catalyzes the methylation of 3,4-dihydroxy-5-hexaprenylbenzoate (DHHB) to 3-methoxy-4-hydroxy-5-hexaprenylbenzoate (HMHB) and the methylation of 2-hexaprenyl-3-methyl-5-hydroxy-6-methoxy-1,4-benzoquinol (3-demethylubiquinol-6) to ubiquinol-6. 3,4-dihydroxy-5-all-trans-polyprenylbenzoate + S-adenosyl-L-methionine = 3-methoxy,4-hydroxy-5-all-trans-polyprenylbenzoate + H(+) + S-adenosyl-L-homocysteine a 3-demethylubiquinol + S-adenosyl-L-methionine = a ubiquinol + H(+) + S-adenosyl-L-homocysteine Regulated in response to catabolite repression. Cofactor biosynthesis; ubiquinone biosynthesis. Component of a multi-subunit COQ enzyme complex, composed of at least COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9 (PubMed:15792955, PubMed:24406904). Interacts directly with COQ4 (PubMed:15792955). Present with 259 molecules/cell in log phase SD medium. Belongs to the class I-like SAM-binding methyltransferase superfamily. UbiG/COQ3 family. Extended N-terminus. Extended N-terminus. Extended N-terminus. Extended N-terminus.