Members of the Alphavirus genus are arboviruses that alternate replication in mosquitoes and vertebrate hosts. In vertebrate
cells, the alphavirus resists the activation of antiviral RNA-activated protein kinase (PKR) by the presence of a prominent RNA
structure (downstream loop [DLP]) located in viral 26S transcripts, which allows an eIF2-independent translation initiation of
these mRNAs. This article shows that DLP structure is essential for replication of Sindbis virus (SINV) in vertebrate cell lines
and animals but is dispensable for replication in insect cells, where no ortholog of the vertebrate PKR gene has been found. Sequence
comparisons and structural RNA analysis revealed the evolutionary conservation of DLP in SINV and predicted the existence
of equivalent DLP structures in many members of the Alphavirus genus. A mutant SINV lacking the DLP structure evolved
in murine cells to recover a wild-type phenotype by creating an alternative structure in the RNA that restored the translational
independence for eIF2. Genetic, phylogenetic, and biochemical data presented here support an evolutionary scenario for the natural
history of alphaviruses, in which the acquisition of DLP structure in their mRNAs probably allowed the colonization of vertebrate
host and the consequent geographic expansion of some of these viruses worldwideThis project was supported in part by the Fundación Mutua Madrileña
(FMM 2008), the VIRUS-HOST Interaction Program (Comunidad de
Madrid), and by a grant from the Ministerio de Ciencia e Innovación
(BFU2010-17411). Institutional support from the Fundación Ramón
Areces is also acknowledge
