Translation from the 5 ' untranslated region (UTR) of mRNA 1 is repressed,but that from the 5 ' UTR of mRNA 7 is stimulated in coronavirus-infected cells

Viral gene products are generally required in widely differing amounts for successful virus growth and assembly. For coronaviruses, regulation of tran scription is a major contributor to these differences, but regulation of tr anslation may also be important. Here, we examine the possibility that the 5' untranslated regions (UTRs), unique for each of the nine species of mRNA in the bovine coronavirus and ranging in length from 70 nucleotides (nt) t o 210 nt (inclusive of the common 5'-terminal 65-nt leader), can differenti ally affect the rate of protein accumulation. When the natural 77-nt 5' UTR on synthetic transcripts of mRNA 7 (mRNA for N and I proteins) was replace d with the 210-nt 5' UTR from mRNA 1 (genomic RNA, mRNA for viral polymeras e), approximately twofold-less N, or (N) CAT fusion reporter protein, was m ade in vitro. Twofold less was also made in vivo in uninfected cells when a T7 RNA polymerase-driven transient-transfection system was used. In corona virus-infected cells, this difference surprisingly became 12-fold as the re sult of both a stimulated translation from the 77-nt 5' UTR and a repressio n of translation from the 210-nt 5' UTR These results reveal that a differe ntial 5' UTR-directed regulation of translation can occur in coronavirus-in fected cells and lead us to postulate that the direction and degree of regu lation is carried out by viral or virally induced cellular factors acting i n trans on cis-acting elements within the 5' UTR.