“…This has been established by studies on the incorporation of I4C-labeled species of guanosine into riboflavin in S. typhimurium (9, P. guillieimondii (13,14) E. ashbyii (7,15), and B. subtilis (8) and decisively confirmed with the discoveries of enzymes from various sources that catalyze the reduction of the nucleotidyl ribose group (13,16,17). The earlier contention that ribitol and ribose are preferentially incorporated into the ribityl group of riboflavin (1 [8][9][10][11][12][13][14][15][16][17][18][19][20] was based on studies with whole cells under conditions in which the effects of pool sizes were hard to interpret and thus the conclusions were suspect. In fact, Miersch, one of the advocates of a precursor role for ribitol (18,19), has recently stated that earlier observations were inconclusive (1 3,2 1) and, with collaborators, has done some of the recent work to indicate that the ribityl group is derived from the ribose of GTP (1 3,2 1).…”