RNA interference (RNAi) is a conserved RNA silencing pathway that leads to sequence-specific mRNA decay in response to the presence of double-stranded RNA (dsRNA). Long dsRNA molecules are first processed by Dicer into 21-22-nucleotide small interfering RNAs (siRNAs). The siRNAs are incorporated into a multimeric RNA-induced silencing complex (RISC) that cleaves mRNAs at a site determined by complementarity with the siRNAs. Following this initial endonucleolytic cleavage, the mRNA is degraded by a mechanism that is not completely understood. We investigated the decay pathway of mRNAs targeted by RISC in Drosophila cells. We show that 5 mRNA fragments generated by RISC cleavage are rapidly degraded from their 3 ends by the exosome, whereas the 3 fragments are degraded from their 5 ends by XRN1. Exosome-mediated decay of the 5 fragments requires the Drosophila homologs of yeast Ski2p, Ski3p, and Ski8p, suggesting that their role as regulators of exosome activity is conserved. Our findings indicate that mRNAs targeted by siRNAs are degraded from the ends generated by RISC cleavage, without undergoing decapping or deadenylation.

Germline mutations of the BRCA1 gene predispose individuals mainly to the development of breast and/or ovarian cancer. However, the exact function of the gene is still unclear, although the encoded proteins are involved in various cellular processes, including transcriptional regulation and DNA repair pathways. Several BRCA1 splice variants are found in different tissues, but in spite of intense investigations, their regulation and possible functions are poorly understood at the moment. This review summarises current knowledge on the roles of these splice variants and the mechanisms responsible for their formation. Because alternative splicing is now widely accepted as an important source of genetic diversity, elucidating the functions of the BRCA1 splice variants would help in the understanding of the exact role(s) of this tumour suppressor. This should help to resolve the current paradox that, despite its seemingly vital cellular functions, mutations of this gene are associated with tissue specific tumour formation predominantly in the breast and the ovary.

Expression of multidrug resistance ABC transporters has been suggested as a functional marker and chemoprotective element in early human progenitor cell types. In this study we examined the expression and function of the key multidrug-ABC transporters, ABCB1, ABCC1 and ABCG2 in two human embryonic stem (HuES) cell lines. We detected a high level ABCG2 expression in the undifferentiated HuES cells, while the expression of this protein significantly decreased during early cell differentiation. ABCG2 in HuES cells provided protection against mitoxantrone toxicity, with a drug-stimulated overexpression of the transporter. No significant expression of ABCB1/ABCC1 was found either in the undifferentiated or partially differentiated HuES cells. Examination of the ABCG2 mRNA in HuES cells indicated the use of selected promoter sites and a truncated 3' untranslated region, suggesting a functionally distinct regulation of this transporter in undifferentiated stem cells. The selective expression of the ABCG2 multidrug transporter indicates that ABCG2 can be applied as a marker for undifferentiated HuES cells. Moreover, protection of embryonic stem cells against xenobiotics and endobiotics may depend on ABCG2 expression and regulation.

Translation-dependent mRNA quality control systems protect the protein homeostasis of eukaryotic cells by eliminating aberrant transcripts and stimulating the decay of their protein products. Although these systems are intensively studied in animals, little is known about the translation-dependent quality control systems in plants. Here, we characterize the mechanism of nonstop decay (NSD) system in Nicotiana benthamiana model plant. We show that plant NSD efficiently degrades nonstop mRNAs, which can be generated by premature polyadenylation, and stop codon-less transcripts, which are produced by endonucleolytic cleavage. We demonstrate that in plants, like in animals, Pelota, Hbs1 and SKI2 proteins are required for NSD, supporting that NSD is an ancient and conserved eukaryotic quality control system. Relevantly, we found that NSD and RNA silencing systems cooperate in plants. Plant silencing predominantly represses target mRNAs through endonucleolytic cleavage in the coding region. Here we show that NSD is required for the elimination of 5′ cleavage product of mi- or siRNA-guided silencing complex when the cleavage occurs in the coding region. We also show that NSD and nonsense-mediated decay (NMD) quality control systems operate independently in plants.