Horizontal gene transfer as a mechanism for the promiscuous acquisition of distinct classes of IRES by avian caliciviruses

摘要

In contrast to members of Picornaviridae which have long 5 '-untranslated regions (5 ' UTRs) containing internal ribosomal entry sites (IRESs) that form five distinct classes, members of Caliciviridae typically have short 5 ' UTRs and initiation of translation on them is mediated by interaction of the viral 5 '-terminal genome-linked protein (VPg) with subunits of eIF4F rather than by an IRES. The recent description of calicivirus genomes with 500-900nt long 5 ' UTRs was therefore unexpected and prompted us to examine them in detail. Sequence analysis and structural modelling of the atypically long 5 ' UTRs of Caliciviridae sp. isolate yc-13 and six other caliciviruses suggested that they contain picornavirus-like type 2 IRESs, whereas ruddy turnstone calicivirus (RTCV) and Caliciviridae sp. isolate hwf182cal1 calicivirus contain type 4 and type 5 IRESs, respectively. The suggestion that initiation on RTCV mRNA occurs by the type 4 IRES mechanism was confirmed experimentally using in vitro reconstitution. The high sequence identity between identified calicivirus IRESs and specific picornavirus IRESs suggests a common evolutionary origin. These calicivirus IRESs occur in a single phylogenetic branch of Caliciviridae and were likely acquired by horizontal gene transfer.