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：

uired for DNA binding, transcriptional activation is mediated exclusively by Rtg3 (Rothermel et al. 1997). This conclusion was derived from the finding that activation of GAL1-lacZ by a Gal4-Rtg1 fusion required functional RTG2 and RTG3 genes, while Gal4-Rtg3 could activate even in an rtg1 and rtg2 background. Rtg3 contains two separate activation domains which map to the N-terminus and C-terminus, respectively, of the protein. A central serine/threonine-rich domain of Rtg3 turned out as inhibitory to transcriptional activation.
 
Fig. 5 Regulatory pathway of retrograde control. Signal-dependent nuclear translocation of the Rtg1+Rtg3 heterodimer is required for activation of R box-dependent target genes. Mks1 acts as an inhibitor of Rtg3, possibly by preventing nuclear translocation of the activator complex. Rtg2 counteracts inhibition by Mks1 by partial dephosphorylation of Rtg3. Rtg2 acts as a sensor of respiratory deficiency and availability of glutamate (both signals may coincide). Tor kinases may also act via Rtg2. The function of the negative factor Lst8 is poorly understood. P Phosphate residue, R box regulatory promoter element of retrograde control (upstream activation site UASr)

While expression of CIT2 and DLD3 shows retrograde control and depends on Rtg regulators under all conditions tested, TCA cycle genes CIT1, ACO1, IDH1 and IDH2 require functional RTG genes in cells whose mitochondrial function has been eliminated (Liu and Butow 1999). These additional genes are not affected by retrograde control but are activated by Hap2–Hap5 in respiration-competent strains. In cells with compromised mitochondrial function, a switch from dependence on Hap activator complex to control by Rtg occurs. Importantly, the corresponding enzymes finally synthesize a-ketoglutarate, which is a metabolic precursor of glutamate. It has been proposed that retrograde control senses mitochondrial dysfunction via the level of glutamate (Sekito et al. 2000). Because glutamate mediates repression of RTG-dependent genes, the retrograde pathway is likely to be important for glutamate homeostasis (Komeili et al. 2000). RTG genes may also be important for activation of the pyruvate carboxylase gene PYC1 (Menendez and Gancedo 1998), contributing to the production of oxaloacetate which allows subsequent conversion into aspartate.

In contrast to R box-binding factors Rtg1 and Rtg3, the function of Rtg2 is less clear. In both q+ and q0 cells, Rtg2 is a cytoplasmic protein with a N-terminal ATPbinding domain and some similarity to bacterial polyphosphatases (according to Liu and Butow 1999). Rtg2 plays a pivotal role as a sensor of the functional state of mitochondria (Komeili et al. 2000; Sekito et al.2000) and also acts in the upstream regulatory pathway of nitrogen catabolism (Pierce et al. 2001). Importantly, activation of RTG-dependent genes correlates with translocation of Rtg1+Rtg3 from the cytoplasm to the nucleus, which requires Rtg2 (Sekito et al. 2000). In rtg2 mutants, Rtg3 is localized constitutively in the cytoplasm, while permanent nuclear localization is found in rtg1 and rtg1 rtg2 mutants. Thus, Rtg1 is both a negative factor of retrograde regulation by retaining Rtg3 in the cytoplasm and a positive factor by allowing Rtg3 binding to R boxes. Cytoplasmic Rtg3 is a phosphoprotein (Sekito et al. 2000). As depicted in Fig. 5, Rtg2 may promote nuclear translocation of Rtg1+Rtg3 by stimulating partial dephosphorylation of Rtg3. RTGdependent genes are negatively regulated by target of rapamycin (Tor) protein kinases, which antagonize nuclear entry of Rtg1+Rtg3. Tor1 and Tor2 function in signaling nutrient availability and antagonize downstream effectors (reviewed by Abraham 2002). Nuclear accumulation of Rtg1+Rtg3 and expression of their target genes is induced in the presence of rapamycin which specifically inhibits Tor kinases and changes the phosphorylation state of Rtg3 (Komeili et al. 2000). Artificial glutamine depletion by treatment of yeast cells with the glutamine synthetase inhibitor L-methionine sulfoximine also caused nuclear localization of Rtg1+Rtg3 (Crespo et al. 2002). Thus, Tor kinases mediate a substantial co-regulation of Rtg activators and general factors of nitrogen catabolite repression (such as Gln3). Nuclear export of Rtg3 requires Msn5.

Rtg2 as a positive factor of retrograde control senses several regulatory inputs (outlined in Fig. 5) and counteracts the Mks1 repressor. Although initially considered as a negative regulator of the Ras-cAMP signaling pathway (Matsuura and Anraku 1993), recent work showed the importance of Mks1 for preventing the nuclear import of Rtg1+Rtg3 (Dilova et al. 2002; Sekito et al. 2002). Mks1 is identical to Lys80, described as a factor required for specific repression of the lysine biosynthetic pathway (Feller et al. 1997). In mks1 and mks1 rtg2 mutant strains, expression of