Academic journal article Genetics

Nitrogen Starvation and TorC1 Inhibition Differentially Affect Nuclear Localization of the Gln3 and Gat1 Transcription Factors through the Rare Glutamine tRNA^sub CUG^ in Saccharomyces Cerevisiae

Academic journal article Genetics

Nitrogen Starvation and TorC1 Inhibition Differentially Affect Nuclear Localization of the Gln3 and Gat1 Transcription Factors through the Rare Glutamine tRNA^sub CUG^ in Saccharomyces Cerevisiae

Article excerpt

MECHANISMS of nitrogen-responsive transcriptional regulation in Saccharomyces cerevisiae and other organisms have remained relatively obscure despite intensive investigation and identification of many required or involved components. The overall complexity of the problem and challenges in elucidating the mechanistic details of overall nitrogen-responsive regulation derive from the fact that four or five distinguishable pathways operate in achieving it (Tate and Cooper 2013). Using Gln3 as the nitrogenresponsive reporter, each mode of regulation was shown to be associated with a distinct physiological condition: (1) short-term nitrogen limitation or growth with poor nitrogen sources, (2) long-term nitrogen starvation, (3) treatment with the glutamine synthetase inhibitor Msx, (4) rapamycin inhibition of TorC1, and (5) leucine starvation or inhibition of leucyl tRNA synthetase.

Gln3 and Gat1 are GATA-family transcription activators that have long been known to be responsible for catabolic nitrogenresponsive or nitrogen catabolite repression (NCR)-sensitive gene expression (Cooper 1982, 2004; Hofman-Bang 1999; Magasanik and Kaiser 2002; Broach 2012; Conrad et al. 2014). When cells are cultured with readily used nitrogen sources (also referred to as good, preferred, repressive, e.g., glutamine), Gln3 is restricted to the cytoplasm, and therefore, the NCR-sensitive transcription it activates is minimal (Cooper 1982). This cytoplasmic sequestration of Gln3 requires the pre-prion protein Ure2 (Blinder et al. 1996; Beck and Hall 1999; Cardenas et al. 1999; Hardwick et al. 1999; Bertram et al. 2000). In contrast, when poorly used nitrogen sources (poor, nonpreferred, derepressive, e.g., proline) are provided, Gln3 relocates to the nucleus, and GATA factor-mediated NCRsensitive transcription increases dramatically.

The five physiological conditions that elicit nuclear entry of Gln3 are distinguished by their protein phosphatase requirements (Tate et al. 2006, 2009, 2010; Georis et al. 2008, 2011; Rai et al. 2013, 2014; Tate and Cooper 2013). Nuclear Gln3 localization in response to short-term nitrogen starvation or growth in a poor nitrogen source requires only Sit4 phosphatase. Nuclear Gln3 localization in response to long-term nitrogen starvation or Msx treatment exhibits no known phosphatase requirement, whereas a response to rapamycin treatment in glutamine-grown cells requires two phosphatases, Sit4 and PP2A (Beck and Hall, 1999, Tate et al. 2006, 2009). Finally, Gln3 localization does not demonstrably respond to leucine/leucyl tRNA synthetase activation of TorC1, which controls Sch9 phosphorylation (Binda et al. 2009; Bonfils et al. 2012; Zhang et al. 2012; Panchaud et al. 2013; Tate and Cooper 2013). Sch9 is a protein kinase that regulates protein synthesis, a major consumer of nitrogenous precursors.

Gat1, a homolog of Gln3 and NCR-sensitive transcription activator in its own right, shares many regulatory characteristics with Gln3. These two GATA factors are not regulated identically, however (Georis et al. 2008, 2011). The most striking difference in the regulation of Gln3 and Gat1 is their responses to Msx and rapamycin. Gln3 is exquisitely sensitive to Msx treatment, whereas Gat1 localization is immune to it (Georis et al. 2011; Tate and Cooper 2013). Conversely, Gat1 is exquisitely sensitive to rapamycin treatment, whereas Gln3 is much less so.

GATA factor localization and function, however, are not the only nitrogen-responsive cellular processes. Others include sporulation, autophagy, and the formation of pseudohyphae in adverse nitrogen conditions (Gimeno et al.1992). In nitrogen-rich conditions, diploid cells are ellipsoidal and bud in a bipolar manner. In contrast, when cultured under nitrogen conditions that verge on starvation, they bud in a unipolar manner that results in the formation of pseudohyphae (Gimeno et al.1992). It has been suggested that pseudohyphal growth may facilitate scavenging for additional sources of environmental nitrogen. …

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