Evaluating the influence of signal transduction pathways on cellulase gene expression

Despite thorough investigation of cellulase induction, transcription, secretion, enzyme function and elucidation of important transcript factors, the role of classical signal transduction pathways in these processes only received little attention for quite some time. Although T. reesei protein kinase C (PKC) was among the first PKCs to be characterized in detail in filamentous fungi (Lendenfeld & Kubicek, 1998; Morawetz et al., 1996), this topic did not become a research focus in this fungus and information on characteristics and regulatory targets of protein kinases and phosphatases of T. reesei is still scarce. Nevertheless, the availability of the sequenced genome (Martinez et al., 2008) enabled an initial evaluation of the signalling inventory of this fungus (Schmoll, 2008), which for example revealed interesting differences to other ascomycetes in the number of available two component histidine kinases and casein kinases.

Additionally, early studies also revealed hints as to an involvement of Ca2+-calmodulin signalling in regulation of plant cell wall degradation (Mach et al., 1998). Although in this study only transcription of a xylanase has been studied, coregulation of cellulases and xylanases under many conditions (Stricker et al., 2008a) suggests that also cellulases might be a target of Ca2+-calmodulin signalling.

Detailed investigations aimed at an evaluation of the role of signal transduction pathways in cellulase regulation started less than 10 years ago. Basis for the first study were two T. reesei strains derived from QM6a. QM9414 represents an early improved cellulase producer strain, which produces higher amounts of cellulases than the wild-type, while QM9978 — resulting from the same mutation cycle — does not. Since the defect of QM9978 was neither found to be a mutation in the protein coding region of the major cellulase genes nor in their promotors (Torigoi et al., 1996; Zeilinger et al., 2000) the assumption was that a crucial signal transduction cascade must be perturbed in this strain. Consequently, a cDNA subtraction method (RaSH, rapid subtraction hybridization) was applied to compare transcription of QM9414 and QM9978 under conditions inducing cellulase expression (Schmoll et al., 2004). Although this study did not reveal the actual defect of QM9978, intriguing new insights into the signals influencing cellulase gene expression were gained, which are still subject to investigation.

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