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Fermentation growth medium- induced pathway

Yeast growth in brewery fermentation proceeds under conditions of repression as a result of the high sugar concentration. The repressive effects appear to be mediated principally via the activity of targets of PKA.. In the exponential phase of growth, as discussed previously, the system is down-regulated and the phenotype is one of low-stress resistance (thinner cell wall, low reserves of trehalose and glycogen, down-regulation of stress-related genes). Under these conditions, exhaustion of an essential nutrient, most likely nitrogen in the case of beer fermentations, causes immediate transition into G0. The targets of PKA are up-regulated and the aforementioned phenotypic changes are reversed. The fact that the same relationship between PKA and growth on non-repressing carbon sources is absent has led to the suggestion that a specific fermentation growth medium signalling pathway (FMG) exists, which is entered whenever there is a complete growth medium containing repressing concentrations of glucose, or a similar carbon source. Thus, there is a common pathway that can be activated by one of a number of essential nutrients. Based on the fact that mutants lacking the regulatory subunit of PKA show the same response, it is considered independent of this kinase but working in parallel with the Ras-cyclic AMP pathway (Conrad et al., 2014). Activation of the FMG pathway requires activation of a cyclic AMP-dependent protein kinase (cAPK). It has been demonstrated that activation of FMG by provision of nitrogen to a glucose-containing medium requires the presence of another protein kinase, termed Sch9, which in turn regulates the activity of cAPK (Crauwells et al., 1997). Sch9 is itself a target for phosphorylation by TORC1 (Urban et al., 2007); in addition, it appears to act directly as an inhibitor of PKA (Zhang et al., 2011), indicating at least dual functions.

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