The BARD1-CstF-50 interaction links mRNA 3' end formation to DNA damage and tumor suppression

The mRNA polyadenylation factor CstF interacts with the BRCA1-associated protein BARD1, and this interaction represses the nuclear mRNA polyadenylation machinery in vitro. Given the suspected role of BRCA1/BARD1 in DNA repair, we tested whether inhibition of mRNA processing is linked to DNA damage. Strikingly, we found that 3' cleavage in extracts from cells treated with hydroxyurea or ultraviolet light was strongly, but transiently, inhibited. Although no changes were detected in CstF, BARD1, and BRCA1 protein levels, increased amounts of a CstF/BARD1/BRCA1 complex were detected. Supporting the physiological significance of these results, a previously identified tumor-associated germline mutation in BARD1 (Gln564His) reduced binding to CstF and abrogated inhibition of polyadenylation. Together these results indicate a link between mRNA 3' processing and DNA repair and tumor suppression.     

RNA聚合酶监视的DNA修复机制

生物体在正常生命过程中面临内/外因来源的DNA损伤,DNA损伤不仅影响基因正确复制,也阻碍其正常转录.为避免DNA损伤带来的灾难性后果,生物体进化出一整套修复机制,以保证复制和转录的正确性、基因组的完整性和遗传的稳定性.本文重点综述了RNA聚合酶监视(RNA polymerase-surveilled,RNAP-S)的DNA修复机制.首先从RNA聚合酶(RNA polymerase,RNAP)的结构出发介绍了RNAP对DNA损伤的感知机制;其次讨论了滞留RNAP的回溯、与其模板DNA的解离以及后续修复机制的启动,真核细胞科凯恩综合征B蛋白(Cockayne syndrome protein B,CSB)及其泛素化和8-氧代鸟嘌呤DNA糖基化酶1 (8-oxoguanine DNA glycosylase1,OGG1)介导的RNAP-S修复;最后探讨了RNAP-S损伤修复的生物学意义并展望其前景.     

Polypyrimidine tract binding protein modulates efficiency of polyadenylation

Polypyrimidine tract binding protein (PTB) is a major hnRNP protein with multiple roles in mRNA metabolism, including regulation of alternative splicing and internal ribosome entry site-driven translation. We show here that a fourfold overexpression of PTB results in a 75% reduction of mRNA levels produced from transfected gene constructs with different polyadenylation signals (pA signals). This effect is due to the reduced efficiency of mRNA 3' end cleavage, and in vitro analysis reveals that PTB competes with CstF for recognition of the pA signal's pyrimidine-rich downstream sequence element. This may be analogous to its role in alternative splicing, where PTB competes with U2AF for binding to pyrimidine-rich intronic sequences. The pA signal of the C2 complement gene unusually possesses a PTB-dependent upstream sequence, so that knockdown of PTB expression by RNA interference reduces C2 mRNA expression even though PTB overexpression still inhibits polyadenylation. Consequently, we show that PTB can act as a regulator of mRNA expression through both its negative and positive effects on mRNA 3' end processing.