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：

Transcription of mitochondrial DNA (mtDNA) is catalyzed by a nucleus-encoded RNA polymerase (RPO41 gene product) similar to bacteriophage polymerases (Masters et al. 1987). In addition, a specificity factor encoded by MTF1 (Lisowsky and Michaelis 1988; Jang and Jaehning 1991) is required for the initiation at a 9-bp motif of mtDNA (ATATAAGTA;Jaehning 1993). Mitochondrial transcription of rRNA-tRNA- and protein-encoding genes increases by a factor of 2–20 in the presence of a nonfermentable substrate (Mueller and Getz 1986; Ulery et al. 1994). Presumably, the carbon source influences mitochondrial gene expression indirectly by a five-fold derepression of the nuclear RPO41 gene (Wilcoxen et al. 1988). In contrast, the specificity factor Mtf1 is not affected by glucose regulation (Ulery et al. 1994).

The CYC1 gene encoding iso-1-cytochrome c of the respiratory chain was shown to be up-regulated by a factor of 5–10 in the presence of a nonfermentable carbon source (Zitomer et al. 1979). Subsequently, the detailed analysis of its control region led to the identification of two distinct regulatory elements, UAS1 and UAS2 (Guarente et al. 1984). UAS1 turned out as the binding site of the heme-dependent transcriptional activator Hap1 (Cyp1) which is also able to interact with an apparently unrelated sequence motif upstream of the iso-2-cytochrome c gene CYC7 (Pfeifer et al. 1987; Creusot et al. 1988). UAS2 activates transcription upon growth with a nonfermentable carbon source, mediated by the heterotetrameric activator complex Hap2/Hap3/ Hap4/Hap5 (Hap2–Hap5, cf. Fig. 2; Olesen et al. 1987;Hahn and Guarente 1988; Forsburg and Guarente 1989;McNabb et al. 1995). Subsequent studies revealed the existence of UAS2-related sequence motifs upstream of a large number of nuclear genes involved in mitochondrial function [e.g. the cytochrome oxidase gene COX6 (Trawick et al. 1989), the cytochrome c1 gene CYT1(Schneider and Guarente 1991; Oechsner et al. 1992), the ubiquinol-cytochrome c oxidoreductase gene QCR8 (De Winde and Grivell 1995) and the citrate synthase gene CIT1 (Rosenkrantz et al. 1994); this work also provides a compilation of additional Hap2–Hap5 binding sites]. The UAS2 core sequence (consensus is TNATTGGT;Forsburg and Guarente 1988) is similar to the CCAAT box which functions as a ubiquitous activation element of a large number of unrelated genes in plant and vertebrate systems (reviewed by Mantovani 1999). Importantly, subunits of yeast and human CCAAT boxbinding factors Hap2–Hap5 and NF-Y (CBF, CP1) are highly related and functionally interchangeable (Chodosh et al. 1988; Sinha et al. 1995). Besides its importance for the control of genes involved in TCA cycle, electron transport and heme biosynthesis, the Hap2–Hap5 complex also affects unrelated metabolic functions, such as ammonium assimilation (Dang et al. 1996). A genomewide analysis of transcripts which are derepressed while cells switch from fermentative to respiratory conditions also led to the identification of genes dependent on Hap2–Hap5(DeRisi et al. 1997).

Hap2 consists of 265 amino acids, but contains an evolutionarily conserved core domain of 60 amino acids which is sufficient for complex assembly and DNAbinding (Olesen and Guarente 1990). The core domain could be further divided into subdomains required for CCAAT box-binding (21 amino acids, in which arginine and histidine residues are functionally important) and interaction with Hap5 [subunit association domain (SAD) of 18 amino acids; Xing et al. 1993, 1994]. It has been suggested that Hap5 mediates interaction between the SADs of Hap2 and Hap3 (McNabb et al. 1995;depicted in Fig. 2). In contrast, Hap4 is not necessary for CCAAT box-binding, but instead contains a transcriptional activation domain (Forsburg and Guarente 1989). In agreement with this finding, deletion of HAP4 could be partially bypassed by a fusion of Hap2 with the activation domain of Gal4 (Olesen and Guarente 1990). Transcriptional activation by Hap4 is mediated by its interaction with the essential Tra1 subunit of the histone acetyltransferase complexes SAGA and NuA4 (Brown et al. 2001). Contact of Hap4 with subunits Swi1, Snf2 and Snf5 of the SWI/SNF complex further emphasizes the importance of chromatin remodeling for the activation of respiratory genes (Neely et al. 2002).

Importantly, HAP4 (but not HAP2, HAP3 or HAP5)may act as a target of carbon source regulation, since its transcription is about 9-fold derepressed under respiratory conditions (Forsburg and Guarente 1989; DeRisi et al. 1997). Although the upstream region of HAP4 contains a canonical Mig1-binding site (at )260; Lundin et al. 1994), evidence argues against its functional importance. Instead, a distinct HAP4 promoter fragment (from )745 to )1006) conferred carbon-source regulation to a heterologous control region. Activation was completely dependent on an intact