Light-dependent signaling by G-proteins in H. jecorina

Transmission of environmental signals within a cell is one of the most important processes an organism applies to survive and successfully compete in its ecological niche. Optimization of these processes leads to faster processing of information with less consumption of cellular resources. In the past few years, we have found that the signaling pathways as light response and heterotrimeric G-protein signaling orchestrate regulation of cellulase gene expression in Hypocrea jecorina (anamorph Trichoderma reesei). Intriguingly, we also found that these pathways are interacting and that light is an important environmental cue for cellulase gene expression. Therefore the question arises how the light signal enters the signaling cascade constituted by the heterotrimeric G- proteins and how it influences their effect. In the course of this project we will tackle this issue by elucidation of the interactions between the components of the signaling cascades of light response and heterotrimeric G-proteins. These interactions as well as the investigation of the influence of the respective components on each other and on our model output pathway of cellulase gene expression will enable us to follow up the effect of the light signal and its transmission trough the heterotrimeric G-Proteins to the cellulase promotors. . Fungi are of high importance for our society, for example concerning the increasing number of immunocompromised patients, who are threatened by pathogenic fungi or as producers of peptaiboles, which are discussed as an alternative to classical antibiotics. Moreover, fungi, especially several Trichoderma spp. play an important role in biocontrol of plant pathogens and can thus serve as biological fungicide. In all these processes, signal transduction proteins are key regulators of their efficiency. Hence a more detailed understanding of signaling in fungi will provide insights into mechanisms of signal transduction not only relevant to cellulase gene expression and biofuel production, but also to elucidate further mechanisms of importance for health and economy.

Cellulose as major component of plant cell walls represents one of the most abundant renewable resources today. The filamentous fungus Trichoderma reesei is adapted to degradation of plant cell walls and produces an efficient enzyme cocktail for this task. In the course of this project we investigated interrelationships between different signaling pathways that lead to altered expression of genes needed for cellulose degradation (mainly cellulases). These signaling pathways transmit signals from the environment of a fungus, like whether it is light or dark, if there are high amounts of nutrients and which kind of nutrients (for example carbon sources) or if chemical components in the vicinity indicate the presence of a competitor or potential mating partner (pheromones). We analyzed gene expression of T. reesei upon growth in light versus darkness on different cellulase inducing, non inducing and repressing carbon sources (cellulose, lactose, sophorose, glucose, glycerol). Thereby we identified genes of importance for signaling of environmental conditions and subsequent regulation of cellulase gene expression. We also tested the functions of photoreceptors and components of the heterotrimeric G-protein pathway which is important for nutrient sensing. Our analyses showed an interconnection of light signaling and nutrient signaling via mutual regulation between photoreceptors and the heterotrimeric G-protein pathway. Interestingly, the genes regulated under conditions which induce cellulase expression in light show differences compared to those in darkness. While the core of cellulase specific genes is regulated in light AND darkness, also differences occur. Hence, reacting to the presence of cellulose is still of major importance to the fungus, but this reaction appears modulated depending on night and day. Thereby, also the intensity of light plays an important role for both transcription of cellulase genes and enzyme expression, as the secretion patterns in light differ from those in darkness. The results of this project showed clearly that fungi not only initiate protective measures against the negative effects of light. They also adjust their metabolism, in our case degradation of cellulose, to day and night. As this phenomenon was not known previously, further investigation of the regulatory mechanisms in light and darkness will reveal interesting new perspectives for industrial applications. These mechanisms once elucidated can be exploited for enhancing biotechnological efficiency of fungal fermentations.