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：

CAT8 gene (see below), indicating a functional link of respiration and gluconeogenesis (Brons et al. 2002). However, no direct binding of Cat8 to this region could be demonstrated. Regulated biosynthesis of Hap4 is clearly important for respiratory control, since expression of the HAP4 coding region under control of the ADH1 promoter allowed a substantial transcription of the Hap2–Hap5-dependent genes QCR8 and CYC1 under glucose-repressing conditions (Blom et al. 2000). As a result, such transformants exhibited a shift to a more oxidative metabolism, accompanied by an increase in biomass yield and a reduced production of ethanol. Similarly, an increased rate of respiration by overexpression of HAP4 could also extend the lifespan of S. cerevisiae cells (Lin et al.2002).

In addition to Hap2–Hap5, other mechanisms of carbon source control of respiratory functions exist. The nuclear gene PET494 encodes a translational regulator of the mitochondrial cytochrome oxidase gene coxIII. PET494 transcription is upregulated by a factor of 4–6 with ethanol as a carbon source but does not require Hap2 or Snf1 (Marykwas and Fox 1989). Control regions of some respiratory genes also contain binding sites for pleiotropic transcription factors, such as Abf1 [COX6 (Trawick et al. 1992), QCR2 (Dorsman and Grivell 1990)] or Cbf1 [CYT1 (Oechsner and Bandlow 1996)]. While Cbf1 obviously does not contribute to the derepression of CYT1, the phosphorylation state of Abf1 (which is affected by the carbon source) correlates with the transcription of COX6 (Silve et al. 1992).

Although transcriptional control of respiratory genes clearly dominates regulation, additional mechanisms exist, such as the differential stability of mRNAs(reviewed by Scheffler et al. 1998). Transcription of the succinate dehydrogenase gene SDH2 depends on Hap2–Hap5 but the half-life of its mRNA also shows a substantial variation (>60 min in the presence of a nonfermentable carbon source, <5 min in glucose;Lombardo et al. 1992). The 5¢ untranslated region (UTR) of SDH2 mRNA contains a determinant controlling its differential turnover in glucose and glycerol. A similar pattern of instability was recently shown for the 5¢ UTRs of SDH1 and SUC2 transcripts (De la Cruz et al. 2002). A functional Xrn1 5¢ exonuclease is required for rapid mRNA degradation (Cereghino et al. 1995). Since the phosphorylation of glucose by any hexokinase and a functional REG1 gene are necessary, at least some of the global regulators of glucose repression also affect the rapid turnover of the SDH2 transcript (Cereghino and Scheffler 1996).

Retrograde gene regulation of respiratory genes Control of mitochondrial activity by the nucleus is apparent from the Hap-dependent regulation of respiratory genes. However, expression of a subset of nuclear genes also responds to the state of mitochondrial function(Parikh et al. 1987). This response (retrograde regulation) allows adaptation of cellular metabolism to a respiratory deficiency which may be caused by the loss of mitochondrial DNA (q0 petite mutants). Retrograde regulation was first shown for transcription of the CIT2 gene (Liao et al. 1991), encoding a peroxisomal isoenzyme of citrate synthase. Compared with a respirationcompetent cell (q+), CIT2 expression increased 6- to 30-fold in a q0 petite strain. It had been suggested that retrograde upregulation of CIT2 and the subsequently elevated activity of peroxisomal citrate synthase might compensate for the reduced flux through the TCA cycle. Later, retrograde control was also shown to affect the DLD3 gene, encoding a cytoplasmic D-lactate dehydrogenase (Chelstowska et al. 1999). The CIT2 upstream region contains duplicated GTCAC core sequences (UASr, or R box) in inverted orientation which synergistically respond to mitochondrial dysfunction (Liao and Butow 1993; Jia et al. 1997). Three mutants defective for R box-dependent gene activation (designated rtg1–rtg3) can be identified (Liao and Butow 1993; Jia et al. 1997). In addition, rtg mutants fail to grow with acetate (but can utilize glycerol) and are auxotrophic for glutamate when cultivated on glucose minimal medium (in both q+ and q0 strains). The corresponding wild-type genes RTG1 and RTG3 encode proteins each with a basic helix-loop-helix (bHLH) and leucine zipper motif (cf. Fig. 2), indicating that they may be responsible for interaction with the R box. Although this sequence does not contain a canonical E box motif (CANNTG), which is common to the recognition site of most bHLH proteins, binding of a Rtg1+Rtg3 heterodimer to R boxes could be shown (Jia et al. 1997). While both Rtg1 and Rtg3 are req