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

Abstract

Oxalate decarboxylase (ODC) catalyses the conversion of oxalic acid to formic acid and CO2in 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 ofDichomitus squalens, and cloned the correspondingDs-odcgene. 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 ofDs-odc. The detected stimulation of ODC activity by oxalic acid is more likely due to enzyme activation than to transcriptional upregulation of theDs-odcgene. Our results support involvement of ODC in primary rather than secondary metabolism in fungi.