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Responding to cold

Several genes involved in yeast cold shock have been identified. These include the NRS1 gene (Lee et al., 1991; Kondo and Inouye, 1992; Kondo et al., 1992), the LOT genes (Zhang et al., 2001), and the cell wall-related TIR genes (Kowalski et al., 1995; Abramova et al., 2001a). Translation efficiency is greatly reduced at low temperatures due to the formation of secondary structures in RNA molecules and the inactivation of ribosomes (Jones and Inouye, 1996). To combat this, an early response to cold involves an increased expression of the majority of the genes involved in rRNA synthesis and processing (Sahara et al., 2002; Schade et al., 2004).

Mga2p, a cold sensor, (Nakagawa et al., 2002) has been demonstrated to activate OLE1 transcription in response to low temperature (Nakagawa et al., 2002), which encodes an intrinsic membrane enzyme important for maintaining optimum levels of membrane fluidity and curvature. Interestingly, a large increase in OLE1 expression has been demonstrated during fermentation (Higgins et al., 2003; James et al., 2003), particularly during the initial stages (Higgins et al., 2003). Brewery fermentation is considered to be a low-temperature environment (Leclaire et al., 2003) where cold-induced Mga2p activation of OLE1 expression is likely to occur.

Further evidence that the membranes are a focus for cold shock responses comes from the observation that trehalose accumulation occurs at temperatures of 10°C and below, and coincides with induction of the trehalose-synthesizing enzymes Tps1p and Tps2p (Kandror et al., 2004). This is believed to be an adaptive response, which increases tolerance to low temperatures as well as freezing.

A remodelling of the cell wall appears to take place when cells are subjected to cold shock, since a mannoprotein, Cwp1p (cell wall protein), is down-regulated and expression of the TIP1, TIR1, TIR2 and TIR4 genes is induced (Abramova et al., 2001a). Since flocculation of lager yeast is increased at lower temperatures (Gonzalez et al., 1996) and the temperature of growth has also been reported to affect the final flocculation capacity (Van Iersel et al., 1998), the TIR genes may be important for this fermentation characteristic.

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