Involved in the biosynthesis of the major light-harvesting pigment bacteriochlorophyll c (BChlc), which confers a significant competitive advantage to green sulfur bacteria living at limiting red and near-infrared light intensities. BchQ is a methyltransferase that adds two consecutive methyl groups to the ethyl carbon at the C-8(2) position of 8,12-diethyl-3-vinylbacteriochlorophyllide d to yield 12-ethyl-8-isobutyl-3-vinylbacteriochlorophyllide d. 8,12-diethyl-3-vinylbacteriochlorophyllide d + S-adenosyl-L-methionine = 12-ethyl-8-propyl-3-vinylbacteriochlorophyllide d + H(+) + S-adenosyl-L-homocysteine 12-ethyl-8-propyl-3-vinylbacteriochlorophyllide d + S-adenosyl-L-methionine = 12-ethyl-8-isobutyl-3-vinylbacteriochlorophyllide d + H(+) + S-adenosyl-L-homocysteine Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Porphyrin-containing compound metabolism; bacteriochlorophyll biosynthesis (light-independent). Cells lacking this gene produce bacteriochlorophyll c (BChlc) that is not methylated at C-8(2) and show a growth rate that decreases to 60% of that of the wild-type at low light intensity. Double mutants lacking both bchR and bchQ produce bacteriochlorophyll c (BChlc) that is not methylated at C-8(2) and C-12(1), and show a growth rate that decreases to 41% of that of the wild-type at low light intensity. Belongs to the radical SAM superfamily.