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The bifurcated stem loop 4 (SL4) is crucial for efficient packaging of mouse mammary tumor virus (MMTV) genomic RNA

Version 2 2018-09-28, 12:58
Version 1 2018-06-22, 05:45
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posted on 2018-09-28, 12:58 authored by Farah Mustafa, Valérie Vivet-Boudou, Ayesha Jabeen, Lizna M. Ali, Rawan M. Kalloush, Roland Marquet, Tahir A. Rizvi

Packaging the mouse mammary tumor virus (MMTV) genomic RNA (gRNA) requires the entire 5ʹ untranslated region (UTR) in conjunction with the first 120 nucleotides of the gag gene. This region includes several palindromic (pal) sequence(s) and stable stem loops (SLs). Among these, stem loop 4 (SL4) adopts a bifurcated structure consisting of three stems, two apical loops, and an internal loop. Pal II, located in one of the apical loops, mediates gRNA dimerization, a process intricately linked to packaging. We thus hypothesized that the bifurcated SL4 structure could constitute the major gRNA packaging determinant. To test this hypothesis, the two apical loops and the flanking sequences forming the bifurcated SL4 were individually mutated. These mutations all had deleterious effects on gRNA packaging and propagation. Next, single and compensatory mutants were designed to destabilize then recreate the bifurcated SL4 structure. A structure-function analysis using bioinformatics predictions and RNA chemical probing revealed that mutations that led to the loss of the SL4 bifurcated structure abrogated RNA packaging and propagation, while compensatory mutations that recreated the native SL4 structure restored RNA packaging and propagation to wild type levels. Altogether, our results demonstrate that SL4 constitutes the principal packaging determinant of MMTV gRNA. Our findings further suggest that SL4 acts as a structural switch that can not only differentiate between RNA for translation versus packaging/dimerization, but its location also allows differentiation between spliced and unspliced RNAs during gRNA encapsidation.

Funding

This research was funded by a joint grant from the United Arab Emirates University (UAEU; Zayed Center for Health Sciences) and Terry Fox Funds for Cancer Research grants (fund codes 31R020 and 21M095 respectively) and College of Medicine & Health Sciences grant (NP-14-34) to TAR and in part from and UAEU-National Research Foundation grant (31M101) and UAEU CMHS grant 31M331 to FM .

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