Summary The transcriptional expression pattern of lignocellulolytic enzyme‐encoding genes in white‐rot fungi differs depending on the culture conditions. Recently, it was shown that 13 putative cellulolytic enzyme‐encoding genes were significantly upregulated in most Pleurotus ostreatus ligninolysis‐deficient mutant strains on beech wood sawdust medium. However, the mechanisms by which this transcriptional shift is triggered remain unknown. In this study, we identified one mechanism. Our previous study implied that histone H3 N‐dimethylation at lysine 4 level possibly affects the shift; therefore, we analysed the expression pattern in the disruptants of P. ostreatus ccl1, which encodes a putative component of the COMPASS complex mediating the methylation. The results showed upregulation of 5 of the 13 cellulolytic enzyme‐encoding genes. We also found that rho1b, encoding a putative GTPase regulating signal transduction pathways, was upregulated in the ccl1 disruptants and ligninolysis‐deficient strains. Upregulation of at least three of the five cellulolytic enzyme‐encoding genes was observed in rho1b‐overexpressing strains but not in ccl1/rho1b double‐gene disruptants, during the 20‐day culture period. These results suggest that Rho1b may be involved in the upregulation of cellulolytic enzyme‐encoding genes observed in the ccl1 disruptants. Furthermore, we suggest that Mpk1b, a putative Agaricomycetes‐specific mitogen‐activated protein kinase, functions downstream of Rho1b.

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