REGULATION OF CELLULASE GENE-EXPRESSION IN THE FILAMENTOUS FUNGUS TRICHODERMA-REESEI

Basic features of regulation of expression of the genes encoding the c ellulases of the filamentous fungus Trichoderma reesei QM9414, the gen es cbh1 and cbh2 encoding cellobiohydrolases and the genes egl1, egl2 and egl5 encoding endoglucanases, were studied at the mRNA level, The cellulase genes were coordinately expressed under all conditions studi ed, with the steady-state mRNA levels of cbh1 being the highest. Solka flee cellulose and the disaccharide sophorose induced expression to a lmost the same level. Moderate expression was observed when cellobiose or lactose was used as the carbon source. It was found that glycerol and sorbitol do not promote expression but, unlike glucose, do not inh ibit it either, because the addition of 1 to 2 mM sophorose to glycero l or sorbitol cultures provokes high cellulase expression levels. Thes e carbon sources thus provide a useful means to study cellulase regula tion without significantly affecting the growth of the fungus. RNA slo t blot experiments showed that no expression could be observed on gluc ose-containing medium and that high glucose levels abolish the inducin g effect of sophorose. The results clearly show that distinct and clea r-cut mechanisms of induction and glucose repression regulate cellulas e expression in an actively growing fungus. However, derepression of c ellulase expression occurs without apparent addition of an inducer onc e glucose has been depleted from the medium. This expression seems not to arise simply from starvation, since the lack of carbon or nitrogen as such is not sufficient to trigger significant expression.