SUMMARYThe complete nucleotide sequence of the genomic RNA of the potyvirus potato virus Y strain N (PVYn) was obtained from cloned cDNAs. This sequence is 9704 nucleotides long and can encode a polyprotein of 3063 amino acids. The positions of the cleavage sites at the N terminus of the capsid and cytoplasmic inclusion proteins have been determined. Other putative protein cleavage sites have been deduced by searching for consensus sequences and by analogy with the polyprotein of the tobacco vein mottling virus and of the tobacco etch virus. Comparison of the PVY polyprotein sequence with that of other potyvirus polyproteins shows similarities in genome organization and a high level of identity along most of the polyprotein, except for the putative proteins flanking the helper component. A search for specific protein motifs has revealed the existence of a potential metal-binding site at the putative N terminus of the helper component in potyviruses. The possible functions of this structure are discussed.

Xylan and cellulose are abundant polysaccharides in vascular plants and essential for secondary cell wall strength. Acetate or glucuronic acid decorations are exclusively found on even-numbered residues in most of the glucuronoxylan polymer. It has been proposed that this even-specific positioning of the decorations might permit docking of xylan onto the hydrophilic face of a cellulose microfibril . Consequently, xylan adopts a flattened ribbon-like twofold screw conformation when bound to cellulose in the cell wall . Here we show that ESKIMO1/XOAT1/TBL29, a xylan-specific O-acetyltransferase, is necessary for generation of the even pattern of acetyl esters on xylan in Arabidopsis. The reduced acetylation in the esk1 mutant deregulates the position-specific activity of the xylan glucuronosyltransferase GUX1, and so the even pattern of glucuronic acid on the xylan is lost. Solid-state NMR of intact cell walls shows that, without the even-patterned xylan decorations, xylan does not interact normally with cellulose fibrils. We conclude that the even pattern of xylan substitutions seen across vascular plants enables the interaction of xylan with hydrophilic faces of cellulose fibrils, and is essential for development of normal plant secondary cell walls.