Seventeen full-length cDNAs encoding pectin methyl esterase I (PME I) have been isolated from the filamentous fungus Aspergillus aculeatus by expression cloning in yeast. Yeast colonies expressing functional PME I were identified on agar plates containing highly esterified pectin, and a cDNA encoding PME I was isolated. The deduced amino acid sequence of PME I is highly similar (74% identity) to the PME from Aspergillus niger. A full-length cDNA encoding PME I was cloned into an Aspergillus expression vector and transformed into Aspergillus oryzae for heterologous expression, purification and characterization of the recombinant enzyme. The recombinant PME I had a molecular mass of 36.2 kDa, an isoelectric point of pH 3.8, a pH optimum of 4.6 and a temperature optimum of 45 degrees C. The authentic PME I was purified from A. aculeatus culture supernatant and subjected to amino acid sequencing. The peptide sequences covered 138 amino acid residues and were in complete agreement with the deduced PME I sequence. Both recombinant and authentic PME I were glycosylated, but the composition of the glycan moieties was different. PME I was able to remove 75-85% of the methyl groups in highly methylated pectin, and it did not remove acetyl groups from acetylated polysaccharides. When the enzyme was added together with polygalacturonases to pectin, a rapid depolymerization was observed. By comparison, polygalacturonases alone showed a very limited degradation of the methylated substrate. This demonstrates that PME I acts in synergy with polygalacturonases in the degradation of plant cell wall pectin.