酿酒酵母非发酵代谢调控
2007-06-17 14:48:04   来源：网络数据库   评论：0 点击：

a switch from high sugar concentrations to growth with a nonfermentable substrate, a 50- to 100-fold increase of transcription occurs for the structural genes ADH2 (encoding the glucose-repressible alcohol dehydrogenase; Denis et al. 1981), ACS1 (acetyl-CoA synthetase; Kratzer and Schu¨ ller 1995), ICL1 (isocitrate lyase; Fernandez et al. 1993; Scho¨ ler and Schu¨ ller 1993),MLS1 (malate synthase; Hartig et al. 1992), PCK1 (phosphoenolpyruvate carboxykinase; Valdes-Hevia et al. 1989) and FBP1 (fructose-1,6-bisphosphatase; Sedivy and Fraenkel 1985). These findings could be confirmed by a genome-wide microarray analysis (DeRisi et al. 1997). Interestingly, the genes of gluconeogenesis are also derepressed upon phagocytosis of S. cerevisiae by murine macrophages (Lorenz and Fink 2001, 2002). This regulation may not simply be caused by a glucosedeprived environment within phagosomes but could also be important for the virulence of pathogenic yeasts. The mortality of mice injected with a mutant Candida albicans strain lacking both ICL1 genes (Dicl1/Dicl1) was clearly reduced compared with a wild-type strain.

Regulation of gluconeogenesis by Cat8 and Sip4
Analysis of the ICL1 control region led to the identification of a UAS element, designated a carbon sourceresponsive element (CSRE; consensus sequence YCCRTTNRNCCG), which was necessary for ICL1 derepression and sufficient to confer carbon sourcedependent transcription on a heterologous reporter gene (Scho¨ ler and Schu¨ ller 1994). CSRE sequence variants could be also found upstream of FBP1 (Niederacher et al. 1993; Hedges et al. 1995; Vincent and Gancedo 1995), PCK1 (Proft et al. 1995), MLS1 (Caspary et al. 1997), MDH2 (Roth and Schu¨ ller 2001), ACS1 (Kratzer and Schu¨ ller 1997), SFC1 (initially designated ACR1 and encoding the succinate/fumarate transporter of the mitochondrial membrane; Palmieri et al. 1997; Bojunga et al. 1998; Redruello et al. 1999), CAT2 (encoding a carnitine acetyltransferase isoenzyme involved in the import of activated acetate into mitochondria; Van den Berg et al. 1998), IDP2 (encoding the NADP-dependent cytoplasmic isocitrate dehydrogenase; Bojunga and Entian 1999) and the lactate permease gene JEN1 (Bojunga and Entian 1999). Two factors which can bind to functional CSRE motifs were identified in protein extracts from derepressed cells but not from cells grown under repressing conditions (Scho¨ ler and Schu¨ ller 1994; Vincent and Gancedo 1995). Importantly, these factors were not detectable in extracts from snf1 or snf4 mutants, grown under either conditions.

Activation of CSRE-dependent structural genes specifically requires a functional CAT8 gene, encoding a protein with a binuclear zinc cluster motif (Zn2Cys6) at its N-terminus (Hedges et al. 1995; Rahner et al. 1996). Thus, Cat8 is a member of the Gal4 family of transcription factors (Fig. 2; summarized by Schjerling and Holmberg 1996) showing significant similarity to the zinc cluster domain found in Sip4 of S. cerevisiae (Lesage et al. 1996). SIP4 was isolated as an interaction partner of the Snf1 protein kinase, supporting the hypothesis of a transcriptional activator involved in carbon-source regulation. The zinc cluster domains of Cat8 and Sip4 are also similar to the KlCat8 of Kluyveromyces lactis (Georis et al. 2000) and FacB of Aspergillus nidulans, required for the activation of genes for acetate utilization (Todd et al. 1997, 1998). Indeed, Cat8 (Rahner et al. 1999) and Sip4 (Vincent and Carlson 1998) were identified as genuine CSRE-binding factors. With both proteins, C-terminal regions are indispensable for transcriptional activation. CAT8 and SIP4 unequally contribute to gene activation via CSREs. While cat8 mutants are unable to grow with a nonfermentable carbon source, due to a substantially reduced activation of CSRE-dependent genes (Hedges et al. 1995; Rahner et al. 1996), growth of sip4 mutants did not differ from wild-type strains (Lesage et al. 1996). Recently, a comparative microarray analysis of transcripts from wild-type and mutant strains confirmed the general importance of Cat8 for gluconeogenesis and related metabolic pathways (Haurie et al. 2001). Interestingly, the KlCat8 of K. lactis is also required for the expression of respiratory genes, such as KlQCR8 (Brons et al.2001).

Two mechanisms of carbon source control could be identified for the transcriptional regulator Cat8 (shown in Fig. 6): (1) in the presence of high glucose, biosynthetic derepression of CAT8 is prevented by the Mig1 repressor (Hedges et al. 1995) and (2) transcriptional activation of a GAL1-lacZ reporter gene by a constitutively produced Gal4-Cat8 fusion protein occurs under derepressing, but not under repressing conditions (Rahner et al. 1996; Randez-Gil et al. 1997). The biosynthetic derepression of Cat8 by deactivation of Mig1 and transcriptional activation mediated by Cat8 requi