| Summary: | <p>Abstract</p> <p>Background</p> <p>Sulphur compounds like cysteine, methionine and S-adenosylmethionine are essential for the viability of most cells. Thus many organisms have developed a complex regulatory circuit that governs the expression of enzymes involved in sulphur assimilation and metabolism. In the filamentous fungus <it>Hypocrea jecorina </it>(anamorph <it>Trichoderma reesei</it>) little is known about the participants in this circuit.</p> <p>Results</p> <p>Analyses of proteins binding to the cellulase activating element (CAE) within the promotor of the cellobiohydrolase <it>cbh2 </it>gene led to the identification of a putative E3 ubiquitin ligase protein named LIMPET (LIM1), which is an orthologue of the sulphur regulators SCON-2 of <it>Neurospora crassa </it>and Met30p of <it>Saccharomyces cerevisiae</it>. Transcription of <it>lim1 </it>is specifically up-regulated upon sulphur limitation and responds to cellulase inducing conditions. In addition, light dependent stimulation/shut down of cellulase gene transcription by methionine in the presence of sulphate was observed. Further, <it>lim1 </it>transcriptionally reacts to a switch from constant darkness to constant light and is subject to regulation by the light regulatory protein ENVOY. Thus <it>lim1</it>, despite its function in sulphur metabolite repression, responds both to light as well as sulphur- and carbon source. Upon growth on cellulose, the uptake of sulphate is dependent on the light status and essential for growth in light. Unlike other fungi, growth of <it>H. jecorina </it>is not inhibited by selenate under low sulphur conditions, suggesting altered regulation of sulphur metabolism. Phylogenetic analysis of the five sulphate permeases found in the genome of <it>H. jecorina </it>revealed that the predominantly mycelial sulphate permease is lacking, thus supporting this hypothesis.</p> <p>Conclusion</p> <p>Our data indicate that the significance of the sulphate/methionine-related signal with respect to cellulase gene expression is dependent on the light status and reaches beyond detection of sulphur availability.</p>
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