Higher plant glycosyltransferases

Abstract

Uridine diphosphate (UDP) glycosyltransferases (UGTs) mediate the transfer of glycosyl residues from activated nucleotide sugars to acceptor molecules (aglycones), thus regulating properties of the acceptors such as their bioactivity, solubility and transport within the cell and throughout the organism. A superfamily of over 100 genes encoding UGTs, each containing a 42 amino acid consensus sequence, has been identified in the model plant Arabidopsis thaliana. A phylogenetic analysis of the conserved amino acids encoded by these Arabidopsis genes reveals the presence of 14 distinct groups of UGTs in this organism. Genes encoding UGTs have also been identified in several other higher plant species. Very little is yet known about the regulation of plant UGT genes or the localization of the enzymes they encode at the cellular and subcellular levels. The substrate specificities of these UGTs are now beginning to be established and will provide a foundation for further analysis of this large enzyme superfamily as well as a platform for future biotechnological applications.

Figure 1

Amino acid consensus sequence of UDP-glycosyltransferases taken from the PROSITE database of protein families and domains, which was used to identify the 107 A. thaliana UGT genes [7]. Letters in brackets denote alternative amino acids at a particular position; X denotes any amino acid.

Figure 2

Summary of the current UGT superfamily nomenclature system. The diagram illustrates the system currently used to name plant UDP glycosyltransferases. Further details of this nomenclature system can be found in [3] and on the UDP Glucuronosyltransferase home page [4].

Figure 3

Phylogenetic analysis of the Arabidopsis UGT superfamily. Neighbor-joining and parsimony-based analysis of nine conserved amino acid sequences shown in Figure 4 was performed as described previously [7]. Bootstrap values over 60% are indicated above the nodes, with the number on the left indicating neighbor-joining and that on the right indicating parsimony. Dashes indicate bootstrap values under 60%. Further refinement of more closely related sequences has been shown in the equivalent analysis of 88 Arabidopsis UGTs [7]. Hypothetical intron gains and losses are indicated by diamonds with the intron number (I) shown (see Figure 4). Postulated intron gains are indicated by filled diamonds, intron losses by unfilled diamonds and the questionable intron loss by a striped diamond.

Figure 4

The conserved regions and intron positions of the UGT genes of A. thaliana. The nine conserved amino acid regions are shown as red boxes. Segments between these boxes represent regions with a variable number of residues. The positions of introns are indicated by arrows and inverted triangles. Examples of UGT genes containing one or more of the nine introns are shown.

Figure 5

The relationship of the groups of A. thaliana UGTs with other published plant UGTs. A simplified version of the A. thaliana UGT phylogenetic tree is shown with other plant UGTs added. The bootstrap values, which give the degree of confidence in the branching pattern presented, are 60-90% unless otherwise stated. The published substrate specificities for UGTs other than A. thaliana are listed to the right of the figure. Full species names referred to in the figure are as follows: Brassica napus, Citrus unshiu, Nicotiana tabacum, Perilla frutescens, Zea mays, Sorghum bicolor, Gentiana triflora, Perilla hybrida, Vitis vinifera, Phaseolus lanatus, Phaseolus vulgaris, Dorotheanthus bellidiformis, Solanum tuberosum.