Size markers (lanes M) are given in nucleotides

Size markers (lanes M) are given in nucleotides. to stimulate decay, Puf3p can still bind its target mRNAs. Together, these experiments provide Ziyuglycoside I insight into the carbon source-specific control of Puf3p activity and how such alterations allow Puf3p to dynamically regulate mitochondrial function. == INTRODUCTION == In eukaryotes, regulation of mRNA decay is critical for controlling protein production not only in somatic cells, but in several stages of development as early as the transition from Ziyuglycoside I maternal to zygotic mRNA expression (1). The Puf family of proteins regulates diverse cellular processes such as gametogenesis, embryonic development and memory formation by promoting translational repression and/or degradation of targeted mRNAs. At the molecular level, Puf p21-Rac1 proteins bind conserved UGU sequences within the 3 untranslated region (3 UTR) of transcripts, typically resulting in the disruption of translation initiation complex interactions or stimulation of mRNA degradation by recruitment of decay complexes (2). In a few instances, Puf proteins can stabilize mRNA targets by promoting translation (3). Several factors contribute to mRNA-specific rates of deadenylation, decapping and subsequent decay, such as the presence of cis-acting regulatory sequences found within the 3 UTR (4,5).Saccharomyces cerevisiaePuf3p specifically binds two CNUGUANAUA elements within theCOX17mRNA 3 UTR and accelerates deadenylation and subsequent decay of the transcript in the presence of dextrose (6,7). One mechanism of action involves recruitment of the Ccr4p-Pop2p-Notp deadenylation complex toCOX17mRNA by direct conversation of Puf3p with Ccr4p (8). Yeast Puf4p and Puf5p demonstrate direct interactions with Pop2p, which together with Ccr4p results in deadenylation of the mRNA target (911). Puf3p most likely recruits decapping factors toCOX17as well, as Puf5p-Pop2p complexes also interact with the decapping factor Dcp1p and the RNA helicase Dhh1p (911). Alternatively, Puf3p can promote rapid deadenylation in a Ccr4p-independent Ziyuglycoside I manner by altering the conformation of the poly(A) binding protein-mRNP structure (8). Structurally, Puf3p-COX17binding interactions are mediated by the repeat domain (RD) region, which is composed of eight imperfect repeats (R1R8), plus short flanking sequences (R1 and R8) (12). Each Puf repeat contains three helices, with repeats stacking to adopt an overall curved structure (1315). Individual RNA bases within the Puf Ziyuglycoside I binding site are recognized by successive repeats through stacking interactions and base-specific contacts (12,1523). Similar to other Puf proteins, Puf3RDp binds the 8 nt UGUANAUA sequence in a one base to one repeat manner, in which the conserved UGU bases bind repeats R8, R7 and R6, respectively (12). Moreover, the specificity ofS. cerevisiaePuf3p binding its mRNA target uses a cytosine located two bases upstream of the conserved UGUA due to a unique binding pocket in the Puf3p RD (12). Global analysis of Puf3p-mRNA interactions reveals that Puf3p physically associates with 220 transcripts, 162 of which are Ziyuglycoside I nuclear-transcribed mRNAs that encode mitochondrial proteins (24). The consensus Puf3p binding motif derived from the global analysis, (C/U)(A/C/U)UGUA(A/U)AUA, is present in 174 of the transcripts physically associated with Puf3p (24) and is enriched in the 3 UTRs of manyS. cerevisiaemRNAs (25). Only one of these transcripts,COX17that encodes a mitochondrial copper shuttle (26,27), has been experimentally validated as a target of Puf3p-mediated decay and exhibits conditional regulation of its stability (6,28). Examination of mitochondrial protein steady-state levels reveals that Puf3p is required for decreasing mitochondrial DNA-encoded Cox2p (29), as well as nuclear-encoded Pet100p, Cox4p (29) and Pet123p (30) levels in dextrose conditions. Additionally, Pet123p levels were increased in a nonfermentable carbon source (30). However, it was unclear if Puf3p modulates the mRNA stabilities of these putative targets in a carbon source-dependent manner. The expression levels of mitochondrial mRNAs made up of the Puf3p binding element are tightly regulated both spatially and temporally within the yeast cell. For example, they are coordinately controlled in response to 750 environmental stresses, including heat shock, starvation and carbon source (25,31). These mitochondrial transcripts are expressed early in the reductive/building phase of the metabolic transcriptional cycle (32). Furthermore, computational analysis using microarray data sets confirms that expression of putative Puf3p targets is usually coordinately downregulated in repressing carbon source conditions such as dextrose, and upregulated in nonrepressing conditions such as galactose, raffinose and ethanol (28). These carbon source-dependent changes in gene expression are consistent with the observation that glycolytic/fermentative metabolism of dextrose results in translational repression of genes required for metabolism of alternative carbon sources such as galactose, ethanol and glycerol (33)..