Substrate-specific transcription of the enigmatic GH61 family of the pathogenic white-rot fungus Heterobasidion irregulare during growth on lignocellulose

The GH61 represents the most enigmatic Glycoside Hydrolase family (GH) regarding enzymatic activity and importance in cellulose degradation. Heterobasidion irregulare is a necrotizing pathogen and white-rot fungus that causes enormous damages in conifer forests. The genome of H. irregulare allowed identification of ten HiGH61 genes. qRT-PCR analysis separate the HiGH61 members into two groups; one that show up regulation on lignocellulosic substrates (HiGH61A, HiGH61B, HiGH61D, HiGH61G, HiGH61H, and HiGH61I) and a second showing either down-regulation or constitutive expression (HiGH61C, HiGH61E, HiGH61F, and HiGH61J). HiGH61H showed up to 17,000-fold increase on spruce heartwood suggesting a pivotal role in cellulose decomposition during saprotrophic growth. Sequence analysis of these genes reveals that all GH61s except HiGH61G possess the conserved metal-binding motif essential for activity. The sequences also divide into groups having either an insert near the N terminus or an insert near the second catalytic histidine, which may represent extensions of the substrate-binding surface. Three of the HiGH61s encode cellulose-binding modules (CBM1). Interestingly, HiGH61H and HiGH61I having CBM1s are up-regulated on pure cellulose. There was a common substrate-specific induction patterns of the HiGH61s with several reference cellulolytic and hemicellulolytic GHs, this taken together with their low transcript levels on media lacking lignocellulose, reflect the concerted nature of cell wall polymer degradation.