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：

synthetase gene ACS1(Kratzer and Schu¨ ller 1997) and the aldehyde dehydrogenase ALD6 (hypothetical; Walther and Schu¨ ller 2001). In addition, Adr1 activates genes of glycerol utilization (Bemis and Denis 1988; Pavlik et al. 1993; see above) and is involved in the derepression of peroxisomal enzymes (Filipits et al. 1993). Adr1 contains four transcriptional activation domains which are functionally redundant (Cook et al. 1994; Young et al. 1998) and allow interaction with basal transcription factors such as TFIIB and TFIID and with coactivators such as subunits Ada2 and Gcn5 of the SAGA complex (Chiang et al. 1996;Komarnitsky et al. 1998).

Some controversy exists on the problem how carbonsource
regulation is exerted via Adr1. Overproduction of Adr1 strongly deregulates its target genes (Denis 1987;Simon et al. 1991). Although biosynthesis of Adr1 may be subject to glucose control, possibly at the translational level (Vallari et al. 1992), more recent evidence argues against the importance of this regulation for differential expression of UAS1-containing genes (Sloan et al. 1999). Substitution of natural Adr1 activation domains by the heterologous VP16 domain led to an Adr1 variant which still conferred carbon-source control on target genes, arguing against the regulatory importance of activation domains. No influence of glucose on nuclear localization of Adr1 could be detected. These results (together with the finding that a chimeric activator retaining merely the authentic Adr1 zinc finger domain together with VP16 sequences could still mediate regulated expression of target genes) suggested thatDNAbinding by Adr1 may be affected by the carbon source (Sloan et al. 1999). Phosphorylation of Adr1 could function as an additional mechanism of control. Similar to Cat8, Adr1 also requires a functional Snf1 protein kinase (Ciriacy 1979; Denis 1987). The isolation of constitutive ADR1c mutants showing substantial derepression, even in the presence of high glucose, initially suggested that phosphorylation of Adr1 by the cAMP-dependent protein kinase (PKA) at a consensus recognition site (Ser230) may inhibit the activation of target genes (Cherry et al. 1989). Such a negative regulation by PKA appeared reasonable, since the addition of glucose leads to a rapid increase in intracellular cAMP (Mbonyi et al. 1990). In contrast to the simple assumption of Adr1 deactivation by PKA, strains lacking PKA activity did not display enhanced ADH2 expression under glucose-growth conditions (Denis et al. 1992). Further experiments showed that glucose can inhibit Adr1-mediated activation independently of cAMPdependent phosphorylation at Ser230 (Dombek et al.1993). Nevertheless, PKA negatively affects ADH2 expression, possibly by reducing the biosynthetic level of Adr1 (Dombek and Young 1997). In addition to PKAmediated repression, negative control by glucose is also dependent on Reg1 (Dombek et al. 1993), acting as the regulatory subunit of the type 1 protein phosphatase Glc7 (Tu and Carlson 1995; see above). Findings on the importance of Glc7 for glucose repression of Adr1-dependent gene activation (Dombek et al. 1999) do not distinguish between direct or indirect influence. It is unclear whether Adr1 is a substrate of the Snf1 protein kinase. Nevertheless, recent results provide evidence for a stimulation of Adr1 binding to chromatin by Snf1 under derepressing conditions, while Glc7+Reg1 inhibit binding in the presence of glucose (Young et al. 2002). Hyperacetylation of histones by deletion of histone deacetylase genes Rpd3 and Hda1 allows increased access of Adr1 to the ADH2 promoter, even under repressing conditions (Verdone et al. 2002). Upon promoter binding by Adr1 containing a functional activation domain, repositioning of two nucleosomes occurs (DiMauro et al.2002).

Induction of genes by oleate
In contrast to lipotrophic yeasts, such as C. maltosa and Yarrowia lipolytica, S. cerevisiae grows poorly with oleic acid as the sole carbon substrate. Nevertheless, even S. cerevisiae is able to utilize oleate as a source of biomass and energy, requiring proliferation of peroxisomes (which are the exclusive site for b-oxidation) and import of b-oxidation enzymes (Kunau 1998; Purdue and Lazarow 2001). Uptake of fatty acids may be mediated by the FAT1 gene product which shows similarity to a mammalian fatty acid transport protein (Faergeman et al. 1997; Zou et al. 2002) and to a very long chain fatty acyl-CoA synthetase (Choi and Martin 1999). Cytoplasmic conversion of fatty acids into acyl-CoA derivatives requires ATP-dependent activation by the acyl-CoA synthetases Faa1 and Faa4 (Faergeman et al.2001). Finally, the entry of long-chain acyl-CoA into peroxisomes is mediated by integral membrane proteins Pxa1 (Pal1, Pat2) and Pxa2 (Pat1), which are subunits of a heterodimeric ATP-binding cassette (ABC) transporter(Hettema et al. 1996; Shani and Valle 1996).

Importantly, the size and number of peroxisomes and the activit