Human fibroblast but not lymphoid cells have unusually long polyadenylated interferon-beta 1 mRNAs

Repression of beta interferon gene expression in virus-infected cells is correlated with a poly(A) tail elongation

Expression of beta interferon (IFN-beta) is transiently induced when Namalwa B cells (Burkitt lymphoma cell line) are infected by Sendai virus. In this study, we found that an elongation of the IFN-beta mRNA could be detected in virus-infected cells and that such a modification was not observed when the IFN-beta transcript was induced by a nonviral agent, poly(I-C). Treatment of the cells with a transcriptional inhibitor (actinomycin D or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) resulted in further elongation of the transcript. Characterization of the elongated IFN-beta transcript by primer extension and RNase H treatment showed that the modification was a result of an elongated poly(A) tail of up to 400 nucleotides. We conclude that the poly(A) tail elongation of the IFN-beta transcript is associated with the viral infection. Furthermore, the presence of the elongated IFN-beta transcript correlated with a decrease of IFN-beta protein in the medium and in cell extracts. Sucrose gradient analysis of cytoplasmic extracts showed that IFN-beta transcripts with elongated poly(A) tails were found in the nonpolysomal fractions, whereas the shorter transcripts could be detected in both polysomal and nonpolysomal fractions. A longer form of the IFN-beta mRNA was also found in the nonpolysomal fractions of cells not treated with transcriptional inhibitors. Thus, the observed regulation of IFN-beta mRNA is not entirely dependent on the inhibition of transcription. To our knowledge, this study provides the first example of a poly(A) tail elongation in somatic cells that negatively influences gene expression in vivo.