Quorum sensing for improved production of industrially useful products from filamentous fungi
Quorum sensing (QS) is a cell density dependant phenomena utilized as a communication process among microorganisms to regulate their physiological responses. QS depends on the production and release of low molecular weight, diffusible chemical signalling molecules, known as quorum sensing molecules (QSM) in the extracellular milieu. When the concentrations of the QSMs reach a critical threshold corresponding to a particular cell density, they bind to a receptor enabling them to function as transcriptional regulators. QS is widely studied in Gram-positive and Gram-negative bacteria in addition to unicellular fungi. Various recent studies report the presence of QS in filamentous fungi. Lipophilic molecules, including lactone-based molecules and the oxygenated poly-unsaturated fatty acids, oxylipins, are the major signalling molecules reported in filamentous fungi. This study correlates the cell-densities of filamentous fungi to the regulation of their different physiological responses. Two fungal species have been investigated. The first is Penicillium sclerotiorum. It was found that addition of ethyl acetate extracts from high cell densities of P. sclerotiorum culture increases sporulation delays the onset of hyphal branching and enhances the production of the secondary metabolite, sclerotiorin in shaken flasks as well as stirred tank bioreactors (STR). GC-MS analysis of the high cell-density extracts revealed several molecules including the oxylipin ricinoleic acid that might be involved in the regulation of the P. sclerotiorum physiological responses via QS. The second species studied was Aspergillus terreus. Here, the role of oxylipins as signalling molecules was investigated. It was found that the supplementation of linoleic acid, as an oxylipin precursor, enhances the production of the secondary metabolite lovastatin in shaken flasks and STRs. Studies using linoleic acid were extended, to investigate its effect on the cytosolic proteome profile of A. terreus. Results showed that several proteins were altered, mainly stress-related proteins and those involved in carbohydrate metabolism. Furthermore, the effect of different oxylipins was investigated on intracellular cAMP levels. It was observed that addition of oxylipins induces a burst in cAMP levels; in particular 9-HpODE, the linoleic acid-derived oxylipin in A. terreus, induces cAMP levels in a dose dependant manner. In silico analysis of A. terreus genome revealed the presence of genes encoding the different components of G-protein/cAMP-mediated signalling. It is speculated that the addition of 9-HpODE activates the signalling mechanism in A. terreus by binding to G-protein coupled receptors. Upon activation, the secondary messenger cAMP is produced. cAMP then induces the expression of different genes, triggering different cellular responses such as sporulation, and secondary metabolism in A. terreus.
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