Résumé : O-Methyltransferases (OMT; EC play an important role in the synthesis of lignin precursors by catalyzing the O-methylation of o-diphenolic substrates such as caffeic acid (CA) and 5-hydroxyferulic acid (50H). Here, we report on the purification of a lignin-specific OMT (38 kDa) from poplar (Populus trichocarpa × P. deltoides). The OMT was purified from xylem by a single affinity chromatography step on adenosine agarose. The enzyme uses both CA and 5OH as substrates. We previously have reported the cloning of a corresponding OMTcDNA [Dumas et al., Plant Physiol. 98 (1992) 796-797]. Expression of this OMTcDNA in Escherichia coli further confirmed the identity of the clone. Genomic hybridization demonstrates the presence of one or two OMT genes per haploid poplar genome. RNA gel blot hybridization shows high levels of steady-state OMTmRNA in the xylem of young poplar trees, as compared to the levels in leaves. © 1993.