Polypyrimidine tract-binding protein binds to the complementary strand of the mouse hepatitis virus 3' untranslated region, thereby altering RNA conformation.

Mouse hepatitis virus (MHV) RNA transcription is regulated mainly by the leader and intergenic (IG) sequences. However, a previous study has shown that the 3' untranslated region (3'-UTR) of the viral genome is also required for subgenomic mRNA transcription; deletion of nucleotides (nt) 270 to 305 from the 3'-UTR completely abolished subgenomic mRNA transcription without affecting minus-strand RNA synthesis (Y.-J. Lin, X. Zhang, R.-C. Wu, and M. M. C. Lai, J. Virol. 70:7236-7240, 1996), suggesting that the 3'-UTR affects positive-strand RNA synthesis. In this study, by UV-cross-linking experiments, we found that several cellular proteins bind specifically to the minus-strand 350 nucleotides complementary to the 3'-UTR of the viral genome. The major protein species, p55, was identified as the polypyrimidine tract-binding protein (PTB, also known as heterogeneous nuclear RNP I) by immunoprecipitation of the UV-cross-linked protein and binding of the recombinant PTB. A strong PTB-binding site was mapped to nt 53 to 149, and another weak binding site was mapped to nt 270 to 307 on the complementary strand of the 3'-UTR (c3'-UTR). Partial substitutions of the PTB-binding nucleotides reduced PTB binding in vitro. Furthermore, defective interfering (DI) RNAs harboring these mutations showed a substantially reduced ability to synthesize subgenomic mRNA. By enzymatic and chemical probing, we found that PTB binding to nt 53 to 149 caused a conformational change in the neighboring RNA region. Partial deletions within the PTB-binding sequence completely abolished the PTB-induced conformational change in the mutant RNA even when the RNA retained partial PTB-binding activity. Correspondingly, the MHV DI RNAs containing these deletions completely lost their ability to transcribe mRNAs. Thus, the conformational change in the c3'-UTR caused by PTB binding may play a role in mRNA transcription.