Order of enzymic incorporation of O-methyl groups into the O-methyl-D-glucose-containing polysaccharide of Mycobacterium smegmatis. A tritium-labelling study.

The order of enzymic incorporation of O-methyl groups into the O-methyl-D-glucose-containing polysaccharide (MGP) of Mycobacterium smegmatis, 3MG(J)----G(I)----G(H) ----G(G)----6MG(F)----(GMG)9(E)----[G(L)----G(D)]----G(C) ----[G(K)----G(B)]----G(A)----Ga, where G is D-glucose, 3MG is 3-O-methyl-D-glucose, 6MG is 6-O-methyl-D-glucose, and Ga is D-glyceric acid, was studied by incubating cultures of M. smegmatis with L-[3H-Me]methionine for various times. MGP was then extracted from the cells, and relative radioactivities of residues D, (E + F)average and J, or of D, E average, F, and J, were determined. Tritium-labelling of these residues increased in the reducing-to-nonreducing residue direction, the steepness of the gradient becoming more shallow with increasing incubation time. The results are consistent with a biosynthetic mechanism that involves sequential addition of O-methyl groups to residues of the pre-formed D-glucan, in the reducing-to-nonreducing residue direction.