Nonsense-mediated mRNA reduction and pre-mRNA processing in drosophila

Abstract

From bacteria to mammalian cells, the presence of a nonsense mutation causes a reduction in the level of the mRNA of the corresponding gene. The reduction is not, contrary to initial expectations, due to a passive mechanism by which non translated mRNAs are degraded; rather it is a active process in which active translation, cis-acting sequences and specific trans-acting factors are required. It is generally accepted that this phenomenon is the consequence of an evolutionary conserved mechanism that evolved to protect cells from the potentially deleterious effect of truncated proteins - this is often referred to as the mRNA surveillance system or nonsense mediated mRNA decay (NMD). This phenomenon has been extensively studied in budding yeast and in mammalian systems and to a lesser extent in C. elegans. In yeast the recognition of the nonsense codon appears to occur during cytoplasmic translation and premature translation termination is thought to activate a specific protein complex - called the surveillance complex - which in tum triggers an accelerated decay of the aberrant mRNA. However, contrary to the expectation that the recognition of the nonsense codon should occur during cytoplasmic translation, several studies in mammalian cells indicate that NMD may take place in the nucleus by a mechanism that is independent of cytoplasmic translation. For example, several reports indicate that this reduction occurs while the mRNA is still associated with the nucleus, and that the stability of the cytoplasmic mRNA is unchanged relative to a wild-type allele. The common view in the field is that these apparently discordant results between NMD in yeast and in mammalian cells will eventually be accommodated in a single model in which translation in the cytoplasm plays a prominent role. For example, a commonly given explanation is that the recognition of the nonsense codon takes place during nuclear export, and it has been implied that the apparent effects on nuclear RNA are in fact triggered by the premature abortion of translation at the cytoplasmic side of the nuclear envelope. However not all the data from mammalian systems can be so easily explained by the above model. For example, several reports indicate that nonsense mutations affect the splicing of the corresponding pre-mRNA, which makes it difficult to imagine how premature translation in the cytoplasm could effect such an early event in mRNA biogenesis