Background：

Ubiquinone， also known as coenzyme Q， or Q， is a critical component of the electron transport pathways of both eukaryotes and prokaryotes (Jonassen and Clarke， 2000 [PubMed 10777520]). This lipid consists of a hydrophobic isoprenoid tail and a quinone head group. The tail varies in length depending on the organism， but its purpose is to anchor coenzyme Q to the membrane. The quinone head group is responsible for the activity of coenzyme Q in the respiratory chain. The S. cerevisiae COQ3 gene encodes an O-methyltransferase required for 2 steps in the biosynthetic pathway of coenzyme Q. This enzyme methylates an early coenzyme Q intermediate， 3，4-dihydroxy-5-polyprenylbenzoic acid， as well as the final intermediate in the pathway， converting demethyl-ubiquinone to coenzyme Q. The COQ3 gene product is also capable of methylating the distinct prokaryotic early intermediate 2-hydroxy-6-polyprenyl phenol.[supplied by OMIM， Mar 2008]