Regulation of translation in L-cells infected with reovirus

It was reported recently that reovirus progeny subviral particles contain an active transcriptase (RNA polymerase) enzyme and inactive or masked guanylyl transferase and methylase enzymes. Another recent report showed that the translational machinery of L-cells, which is normally "cap" dependent, becomes modified and undergoes a transition to cap independence as a function of time post-infection. In the present work we have examined the 5′-terminal structure of messenger RNA associated with polyribosomes of L-cells infected with reovirus at early, intermediate and late times post-infection. The results show that reovirus mRNA molecules isolated from polyribosomes at early times contain predominantly, the 5′-terminal structure m⁷G(5′)ppp(5′)G^(m)pCp ... or type-1 cap; mRNA isolated at intermediate times contains two types of mRNA molecules, one terminating in a type-1 cap and the other having an uncapped 5′ terminus whose structure is pG ...; at late times, essentially all viral mRNA is mono-phosphorylated at the 5′ terminus and is therefore uncapped. This pG ...-terminated mRNA is not a substrate for the host capping enzymes, it cannot be translated in vitro in extracts from uninfected cells, but can be translated with high efficiency in extracts prepared from infected cells. L-cell mRNA molecules present in infected cells at late times do not seem to be degraded (at least not extensively) and can be translated in vitro in extracts from uninfected L-cells. These and other results are consistent with the idea that reovirus induces a gradual modification in the cap dependence of the host translational apparatus. As a result of this modification, uncapped reovirus mRNA is translated preferentially over host (capped) mRNA. These data have been drawn together in the form of a revised model for the reovirus replicative cycle.