Primary microRNA transcript retention at sites of transcription leads to enhanced microRNA production

MicroRNAs (miRNAs) are noncoding RNAs with important roles in regulating gene expression. In studying the earliest nuclear steps of miRNA biogenesis, we observe that primary miRNA (pri-miRNA) transcripts retained at transcription sites due to the deletion of 3′-end processing signals are converted more efficiently into precursor miRNAs (pre-miRNAs) than pri-miRNAs that are cleaved, polyadenylated, and released. Flanking exons, which also increase retention at transcription sites, likewise contribute to increased levels of intronic pri-miRNAs. Consistently, efficiently processed endogenous pri-miRNAs are enriched in chromatin-associated nuclear fractions. In contrast, pri-miRNAs that accumulate to high nuclear levels after cleavage and polyadenylation because of the presence of a viral RNA element (the ENE of the Kaposi's sarcoma–associated herpes virus polyadenylated nuclear RNA) are not efficiently processed to precursor or mature miRNAs. Exogenous pri-miRNAs unexpectedly localize to nuclear foci containing splicing factor SC35; yet these foci are unlikely to represent sites of miRNA transcription or processing. Together, our results suggest that pri-miRNA processing is enhanced by coupling to transcription

Jubilee Lecture Jubilee Lecture miRNPs: versatile regulators of gene expression in vertebrate cells 1

Abstract TNFα (tumour necrosis factor α) mRNA bears in its 3 -UTR (untranslated region) a conserved ARE (AU-rich element), a signal that exerts tight post-transcriptional control over the expression of TNFα and other cytokines. We found that the TNFα ARE increases translational efficiency when cell growth is arrested, a physiologically relevant state occurring during inflammation, angiogenesis and monocyte differentiation. Under these conditions, called quiescence, the miRNP (microribonucleoprotein)-associated proteins FXR1 (Fragile X mental retardation-related protein 1) and AGO2 (Argonaute 2), which are usually considered negative regulators, are transformed into effector molecules that bind the ARE to activate translation. We then identified a specific miRNA (microRNA) that directs the association of AGO2 and FXR1 with the ARE during translational up-regulation. Two other wellcharacterized miRNAs likewise promote translation activation in quiescent or in contact-inhibited cells; yet, they repres

Evidence for reassociation of RNA-binding proteins after cell lysis: Implications for the interpretation of immunoprecipitation analyses

Immuno- and other affinity-purification approaches are commonly used to characterize the composition of ribonucleoprotein complexes. While associations detected by these procedures are often interpreted as reflecting in vivo interactions, it is also possible that they arise from reassociation of molecules after cell lysis. Here we used an experimental approach that allowed us to distinguish between these possibilities. Surprisingly, we show that the association of the RNA-binding protein HuR with its target mRNA, c-fos, as detected by co-immunoprecipitation, results largely from reassociation of molecules subsequent to cell lysis. The existence of such postlysis reassortments thus demonstrates that co-immunoprecipitation does not always recapitulate the in vivo state of ribonucleoprotein complexes

Premature termination codons do not affect the rate of splicing of neighboring introns

Introduction of a premature termination codon (PTC) into an exon of a gene can lead to nonsense-mediated decay of the mRNA, which is best characterized as a cytoplasmic event. However, increasing evidence has suggested that PTCs may also influence the nuclear processing of an RNA transcript, leading to models of nuclear surveillance perhaps involving translating nuclear ribosomes. We used quantitative RT-PCR to measure the in vivo steady-state levels of every exon-intron junction in wild-type, PTC-containing, and missense-containing precursor mRNAs of both the nonrearranging dihydrofolate reductase (DHFR) and the somatically rearranging Ig-μ genes. We find that each exon-intron junction’s abundance and, therefore, the rate of intron removal, is not significantly affected by the presence of a PTC in a neighboring exon in either the DHFR or Ig-μ pre-mRNA. Similarly, the abundance of the uncleaved Ig-μ polyadenylation sites does not differ between wild-type and PTC-containing Ig-μ pre-mRNAs. Our Ig-μ data were confirmed by RNase protection analyses, and multiple cell isolates were examined to resolve differences with previously published data on steady-state pre-mRNA levels. We conclude that the presence of a PTC affects the rate of neither splicing nor the cleavage step of 3′ end formation during pre-mRNA processing in the nucleus. Our results are discussed with respect to existing evidence for nuclear surveillance mechanisms