Human immunodeficiency virus type 1 (HIV) gag/pol and env mRNAs contain cis-acting regulatory elements (INS) that impair stability, nucleocytoplasmic transport, and translation by unknown mechanisms. This downregulation can be counteracted by the viral Rev protein, resulting in efficient export and expression of these mRNAs. Here, we show that the INS region in HIV-1 gag mRNA is a high-affinity ligand of p54nrb/PSF, a heterodimeric transcription/splicing factor. Both subunits bound INS RNA in vitro with similar affinity and specificity. Using an INS-containing subgenomic gag mRNA, we show that it specifically associated with p54nrb in vivo and that PSF inhibited its expression, acting via INS. Studying the authentic HIV-1 mRNAs produced from an infectious molecular clone, we found that PSF affected specifically the INS-containing, Rev-dependent transcripts encoding Gag-Pol and Env. Both subunits contained nuclear export and nuclear retention signals, whereas p54nrb was continuously exported from the nucleus and associated with INS-containing mRNA in the cytoplasm, suggesting its additional role at late steps of mRNA metabolism. Thus, p54nrb and PSF have properties of key factors mediating INS function and likely define a novel mRNA regulatory pathway that is hijacked by HIV-1.Many eukaryotic mRNAs are subject to regulated turnover via cis-acting signals which are recognized by trans-acting factors. The best-studied example is the cytoplasmic mRNA decay mediated by cis-acting AU-rich RNA elements and trans-acting ARE-binding proteins. Another important mechanism is nonsense-mediated mRNA decay, which serves to eliminate the transcripts that contain premature stop codons (for recent reviews, see references 17, 34, and 40).In contrast to cytoplasmic pathways, nuclear mRNA turnover is less well understood. Among the mRNAs that are subject to nuclear downregulation are the Rev-responsive element (RRE)-containing mRNAs of human immunodeficiency virus type 1 (HIV-1) (11,22,25,26,32,33,42,48,54; reviewed in references 13, 21, 27, and 47). These unspliced and nonterminally spliced transcripts need to be exported from the nucleus before completion of splicing to produce the Gag-Pol and Env proteins. This regulatory step is accomplished by the viral Rev protein, which binds to the RRE and links these transcripts to the CRM1 export receptor. In the absence of Rev, these mRNAs are further spliced to completion or degraded. However, even when devoid of splice sites, the unspliced or nonterminally spliced mRNAs are poorly expressed due to the presence of cis-acting instability elements (INS/ CRS) that are scattered throughout the gag/pol and env mRNAs (11,25,32,42,50,51,54). These elements act at several steps, impairing mRNA stability, nucleocytoplasmic transport, and translation (5,15,22,29,33,51,54), whereas Rev counteracts these defects, resulting in efficient expression. However, Rev is unable to export nonterminally spliced mRNAs that do not contain a functional INS (51), and hence, INS are an integral part of Rev regula...

RNA metabolism is a major contributor to the pathogenesis of clinical disorders associated with premutation size alleles of the fragile X mental retardation (FMR1) gene. Herein, we determined the structural properties of numerous FMR1 transcripts harboring different numbers of both CGG repeats and AGG interruptions. The stability of hairpins formed by uninterrupted repeat-containing transcripts increased with the lengthening of the repeat tract. Even a single AGG interruption in the repeated sequence dramatically changed the folding of the 5′UTR fragments, typically resulting in branched hairpin structures. Transcripts containing different lengths of CGG repeats, but sharing a common AGG pattern, adopted similar types of secondary structures. We postulate that interruption-dependent structure variants of the FMR1 mRNA contribute to the phenotype diversity, observed in premutation carriers.

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