mRNA export correlates with activation of transcription in human subgroup C adenovirus-infected cells

To investigate the mechanisms by which viral mRNA species are distinguished from their cellular counterparts for export to the cytoplasm during the late phase of subgroup C adenovirus infection, we have examined the metabolism of several cellular and viral mRNAs in human cells productively infected by adenovirus type 5 (Ad5). Several cellular mRNAs that were refractory to, or could escape from, adenovirus-induced inhibition of export of mRNA from the nucleus have been identified. This group includes Hsp70 mRNAs synthesized upon heat shock of Ad5-infected 293 or HeLa cells during the late phase of infection. However, successful export in Ad5-infected cells is not a specific response to heat shock, for beta-tubulin and interferon-inducible mRNAs were also refractory to virus-induced export inhibition. The export of these cellular mRNAs, like that of viral late mRNAs, required the E1B 55-kDa protein. Export to the cytoplasm during the late phase of Ad5 infection of several cellular mRNAs, including members of the Hsp70 family whose export was inhibited under some, but not other, conditions, indicates that viral mRNA species cannot be selectively exported by virtue of specific sequence or structural features. Cellular and viral late mRNAs that can be exported from the nucleus to the cytoplasm were expressed from genes whose transcription was induced or activated during the late phase of Ad5 infection. Consistent with the possibility that successful export is governed by transcriptional activation in the late phase of adenovirus infection, newly synthesized viral early E1A mRNA was subject to export inhibition during the late phase of infection.