Studies of the Nicotiana plumbaginifolia T-DNA nolac-H18 callus mutant lead to the iden­tification of the mutated gene, NpGUT1. NpGUT1 has 60% sequence homology to animal glucuronosyltransferases that synthesize heparin sulfate. Complementation of the nolac — H18 mutant with the NpGUT1 gene corrected the non-organogenesis and weak intercellular attachment phenotypes of the mutant. Cell walls of the nolac-H18 mutant contained 86% reduced levels of glucuronic acid, a reduction that was associated with the pectin-enriched fraction of the walls. RG-II from mutant walls was devoid of glucuronic acid, leading the authors to propose that NpGUT1 encodes RG-II-p-1,2GlcAT that transfers GlcAonto the l — fucose in RG-II side chain A (181). The mutant RG-II in the nolac-H18 showed 82% reduced

RG-II dimer formation, providing further support that RG-II is modified in the mutant. Taken together these results show that NpGUTl encodes a putative RG-II: p-1,2GlcAT. En­zymatic confirmation of the activity of the encoded protein has not yet been reported.

5.4.9.2 RG-II:methyltransferase (RG-II:MT)

RG-II contains 2-O-methylfucose and 2-O-methylxylose. It is not known if the methyl group is added at the stage of the nucleotide-sugar or after the sugar is added to RG-II. The genes encoding the methyltransferases have not been identified.

A pectin methyltransferase activity detergent-solubilized from suspension-cultured flax cells was able to transfer methyl groups from S-adenosylmethionine onto RG-II isolated from wine (334). Enzyme reactions containing RG-II had sevenfold great methyltransferase activity than reactions without exogenous acceptor and the radiolabeled product synthesized had a size similar to RG-II monomers and RG-II dimers (158, 334). It was not established where in RG-II the methyl group was added and thus, the methylation may have represented methylesterification of the HG backbone of RG-II, or alternatively, could have been due to methyletherification of RG-II since RG-II contains methyl groups on non-galacturonic glycosyl residues (e. g., 2- O-methyl xylose and 2- O-methyl fucose (352, 360) of side chain residues. The location of the methylation in RG-II and the identity of the potentially novel enzyme activity that catalyzes its incorporation into RG-II have not been established.