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Oxalate decarboxylase of the white-rot fungus Dichomitus squalens demonstrates a novel enzyme primary structure and non-induced expression on wood and in liquid cultures

Department of Applied Chemistry and Microbiology, Division of Microbiology, Viikki Biocenter, PO Box 56, FIN-00014 University of Helsinki, Finland

Oxalate decarboxylase (ODC) catalyses the conversion of oxalic acid to formic acid and CO₂ in bacteria and fungi. In wood-decaying fungi the enzyme has been linked to the regulation of intra- and extracellular quantities of oxalic acid, which is one of the key components in biological decomposition of wood. ODC enzymes are biotechnologically interesting for their potential in diagnostics, agriculture and environmental applications, e.g. removal of oxalic acid from industrial wastewaters. We identified a novel ODC in mycelial extracts of two wild-type isolates of Dichomitus squalens, and cloned the corresponding Ds-odc gene. The primary structure of the Ds-ODC protein contains two conserved Mn-binding cupin motifs, but at the N-terminus, a unique, approximately 60 aa alanine-serine-rich region is found. Real-time quantitative RT-PCR analysis confirmed gene expression when the fungus was cultivated on wood and in liquid medium. However, addition of oxalic acid in liquid cultures caused no increase in transcript amounts, thereby indicating a constitutive rather than inducible expression of Ds-odc. The detected stimulation of ODC activity by oxalic acid is more likely due to enzyme activation than to transcriptional upregulation of the Ds-odc gene. Our results support involvement of ODC in primary rather than secondary metabolism in fungi.

Correspondence
Miia R. Mäkelä
miia.r.makela{at}helsinki.fi

The GenBank/EMBL/DDBJ accession numbers for the sequences determined in this work are FM946037, FM955140, FM946036, and FM954981 for odc, cDNA of odc, partial gapdh of D. squalens FBCC312, and odc of D. squalens FBCC184, respectively.