RG-II contains 3-O-acetylaceric acid and acetylated methyl fucose. No gene for RG-II acetyltransferase has been identified. However, O-acetyltransferase activity in microsomes from suspension-cultured potato cells (339) has been shown to transfer [14C]acetate from [14C]acetyl-CoA onto endogenous acceptors in the microsomes to yield a salt/ethanol pre- cipitable product from which approximately 8% of the radioactivity could be solubilized by treatment with endopolygalacturonase and pectin methylesterase. Thus, it is possible that the radiolabeled acetate solubilized by the glycanase treatments represented acetylated RG-II and that the activity identified was RG-II acetyltransferase.

5.4.9.3 Other RG-II transferases

The glycosyltransferases that insert fucose, KDO, DHA, and aceric acid into RG-II have not been identified. A 10-member Arabidopsis gene family (118) that has 35-73.8% amino acid sequence identify to an Arabidopsis a 1,2-fucosyltranferase that fucosylates a side branch in the hemicellulose xyloglucan has been described (116). Whether one or more of these genes encodes fucosyltransferase(s) involved in RG-II synthesis remains to be investigated.