Rhamnose is a major sugar moiety in pectin and in various glycosides of secondary metabo­lites. UDP-Rha is the activated sugar for the synthesis of flavonoids (448); however, the activated rhamnose-donor form for pectin synthesis has not been determined. Previously, it was suggested that synthesis of UDP-Rha from UDP-Glc is mediated by three separate enzymes, similar to the conversion of TDP-Glc to TDP-Rha in bacteria (403). UDP-Glc is first modified to the UDP-4-keto-6-deoxyGlc intermediate by UDP-Glc 4,6-dehydratase. The intermediate is modified in the presence of NAD(P)H by a 3,5-epimerase and 4,6- keto-reductase to form UDP-L-p-Rha. A debate in the literature as to whether two or three different enzymes are involved in UDP-Rha came to an end with the functional cloning NRS/er (134) from Arabidopsis (At1g6300). The activity of recombinant NRS/er demon­strates irrefutably that the 3,5-epimerase and 4,6-keto-reductase activities reside in one polypeptide. Interestingly, in vitro NRS/er accepts both TDP — and UDP-4-keto-6-deoxyGlc as substrates to form TDP-Rha and UDP-Rha, respectively. Although TDP-Glc is found in plants (403) and several enzymes can generate TDP-Glc in vitro, the physiological signif­icance of the ability of NRS/er to generate TDP-Rha is unclear. Only by isolating a pectin rhamnosyltransferase and characterizing the donor specificity, can the true nature of NDP — Rha form be conclusively determined.

The Arabidopsis genome consists of three genes (At1g78570, At3g14790, At1g53500) each that encodes a large protein (~670 aa) having two domains: an N-terminal domain (~330 aa long) that shares amino acid sequence similarity to 4,6-dehydratase followed by a C-terminal domain (~320 aa long) that shares over 80% sequence identity to NRS/ER. The C-terminal domain of At1g78570 has similar enzyme activity as NRS/ER (134). Mutations in At1g53500, mum4 (449), and rhm2 (450) result in decreasing amounts of Rha and GalA sugar moieties in RG-I structures isolated from seed mucilage. These mutants provided the first genetic evidence for the involvement of these genes (we named URS, UDP-Rha- synthase) in rhamnose synthesis. More recently, when theses genes were recombinantly expressed in yeast, Oka and coworkers (451) reported that all of the Arabidopsis URS genes (also named RHM/MUM) have UDP-rhamnose synthase activity and interestingly, are highly inhibited by UDP-Xylose.