植物转录抑制子的结构特征及其作用机理

转录因子依转录调控能力可分为激活子和抑制子。植物转录抑制蛋白的分类依据很多,从作用方式上可分为主动抑制子和被动抑制子两大类：根据与DNA结合的方式则可分为锌指类、MYB类、AP2／EREBP类、bHLH类和bZlP类等。植物主动抑制子通过其含有的抑制域对转录直接起抑制作用。抑制域又可分很多类,但多数为含有类似EAR基序的保守性基序,其上具有几个保守性亮氨酸残基。植物转录抑制子主要通过对激活子或基本转录复合物产生作用及改变染色体结构3种方式来抑制目标基因的转录。有关植物转录抑制子的研究虽很欠缺,但以拟南芥SUPERMAN等抑制子的EAR基序为代表的研究表明,抑制域是阐明植物转录抑制子功能和下游基因表达调控机理的核心对象,而融合抑制子沉默技术（CRES-T）也为人为调控基因沉默带来了新的技术手段。     

Overlapping but distinct RNA elements control repression and activation of nanos translation

Spatially restricted synthesis of Nanos protein in the Drosophila embryo is essential for anterior-posterior patterning. Nanos translation is restricted to the posterior of the embryo by translational repression of nanos mRNA throughout the bulk cytoplasm and selective activation of posteriorly localized nanos mRNA. A 90-nucleotide translational control element (TCE) mediates translational repression. We show that TCE function requires formation of a bipartite secondary structure that is recognized by Smaug repressor and at least one additional factor. We also demonstrate that translational activation requires the interaction of localization factors with sequences that overlap TCE structural motifs. The identification of separate but overlapping recognition motifs for translational repressors and localization factors provides a molecular mechanism for the switch between translational repression and activation.     

The unconserved groucho central region is essential for viability and modulates target gene specificity

Groucho (Gro) is a Drosophila corepressor required by numerous DNA-binding repressors, many of which are distributed in gradients and provide positional information during development. Gro contains well-conserved domains at its N- and C-termini, and a poorly conserved central region that includes the GP, CcN, and SP domains. All lethal point mutations in gro map to the conserved regions, leading to speculation that the unconserved central domains are dispensable. However, our sequence analysis suggests that the central domains are disordered leading us to suspect that the lack of lethal mutations in this region reflects a lack of order rather than an absence of essential functions. In support of this conclusion, genomic rescue experiments with Gro deletion variants demonstrate that the GP and CcN domains are required for viability. Misexpression assays using these same deletion variants show that the SP domain prevents unrestrained and promiscuous repression by Gro, while the GP and CcN domains are indispensable for repression. Deletion of the GP domain leads to loss of nuclear import, while deletion of the CcN domain leads to complete loss of repression. Changes in Gro activity levels reset the threshold concentrations at which graded repressors silence target gene expression. We conclude that co-regulators such as Gro are not simply permissive components of the repression machinery, but cooperate with graded DNA-binding factors in setting borders of gene expression. We suspect that disorder in the Gro central domains may provide the flexibility that allows this region to mediate multiple interactions required for repression.     

植物转录抑制子的结构特征及其作用机理

转录因子依转录调控能力可分为激活子和抑制子。植物 转录抑制蛋白的分类依据很多, 从作用方式上可分为主动抑制子和被动抑制子两大类; 根据与DNA结合的方式则可分为锌指类、MYB类、AP2/EREBP类、bHLH类和bZIP类等。植物主 动抑制子通过其含有的抑制域对转录直接起抑制作用。抑制域又可分很多类, 但多数为含有类似EAR基序的保守性基序, 其上具有几个保守性亮氨酸残基。植物转录抑制子主要通过对激活子或基本转录复合物产生作用及改变染色体结构3种方式来抑制目标基因的转录。有关植物转录抑制子的研究虽很欠缺, 但以拟南芥SUPERMAN等抑制子的EAR基序为代表的研究表明, 抑制域是阐明植物转录抑制子功能和下游基因表达调控机理的核心对象, 而融合抑制子沉默技术(CRES-T)也为人为调控基因沉默带来了新的技术手段。     

Two yeast PUF proteins negatively regulate a single mRNA

mRNA stability and translation are regulated by protein repressors that bind 3'-untranslated regions. PUF proteins provide a paradigm for these regulatory molecules: like other repressors, they inhibit translation, enhance mRNA decay, and promote poly(A) removal. Here we show that a single mRNA in Saccharomyces cerevisiae, encoding the HO endonuclease, is regulated by two distinct PUF proteins, Puf4p and Mpt5p. These proteins bind to adjacent sites and can co-occupy the mRNA. Both proteins are required for full repression and deadenylation in vivo; their removal dramatically stabilizes the mRNA. The two proteins act through overlapping but non-identical mechanisms: repression by Puf4p is dependent on deadenylation, whereas repression by Mpt5p can occur through additional mechanisms. Combinatorial action of the two regulatory proteins may allow responses to specific environmental cues and be common in 3'-untranslated region-mediated control.