Characterization of PIR1, a GATA family transcription factor involved in iron responses in the white-rot fungus Phanerochaete chrysosporium

Abstract

Iron, although toxic in excess, is an essential element for biological systems. Therefore, its homeostasis is of critical importance and tight mechanisms participate in its acquisition by microbial organisms. Lately, the relevance of this metal for biomass conversion by wood-degrading fungi has been gaining increasing attention. Iron plays a critical role as cofactor of key enzymes such as lignin and manganese peroxidases in lignin-degrading white-rot fungi, while Fe(II) also serves a pivotal role in Fenton reactions that are central in cellulose depolymerization by brown-rotters. It has been hypothesized that multicopper oxidases with ferroxidase activity might participate in controlling the bioavailability of iron in the hyphal proximity, fine-tuning Fenton chemistry and balancing lignin versus cellulose degradation. In order to further explore the dynamics of iron regulation in the well known white-rot fungus Phanerochaete chrysosporium, we analyzed the mRNA levels of the multicopper oxidases genes (mcos) in response to iron supplementation, confirming down-regulation of their expression in response to this metal. To gain a better understanding on the transcriptional mechanisms mediating this effect, we searched for a gene encoding a GATA-type transcription factor with homology to URBS1, the major transcriptional regulator of iron homeostasis in Ustilago maydis. Due to the limitation of experimental tools in P. chrysosporium, the alleged Phanerochaete iron regulator (PIR1) was studied by complementation of a Neurospora SRE/URBS1-deficient strain, where phenotypic and molecular characteristics of this transcriptional regulator could be easily assessed. In addition, using a genome-wide in silico strategy, we searched for putative cis-acting iron-responsive elements in P. chrysosporium. Some of the identified genes showed reduced transcript levels after 30min in the presence of the metal, consistent with an SRE/URBS1-mediated mechanism, and suggesting a broad effect of iron on the regulation of several cellular processes.