A Tobamovirus Genome That Contains an Internal Ribosome Entry Site Functionalin Vitro

Abstract

Most eukaryotic mRNAs are translated by a “scanning ribosome” mechanism. We have found that unlike the type member of the genusTobamovirus,translation of the 3′-proximal coat protein (CP) gene of a crucifer infecting tobamovirus (crTMV) (Dorokhovet al.,1993; 1994) occurredin vitroby an internal ribosome entry mechanism. Three types of synthetic dicistronic RNA transcripts were constructed and translatedin vitro:(i) “MP-CP-3′NTR” transcripts contained movement protein (MP) gene, CP gene and the 3′-nontranslated region of crTMV RNA. These constructs were structurally equivalent to dicistronic subgenomic RNAs produced by tobamovirusesin vivo.(ii) “ΔNPT-CP” transcripts contained partially truncated neomycin phosphotransferase I gene and CP gene. (iii) “CP-GUS” transcripts contained the first CP gene and the gene ofEscherichia coliβ-glucuronidase (GUS) at the 3′-proximal position. The results indicated that the 148-nt region upstream of the CP gene of crTMV RNA contained an internal ribosome entry site (IRESCP) promoting internal initiation of translationin vitro.Dicistronic IRESCP, containing chimeric mRNAs with the 5′-terminal stem–loop structure preventing translation of the first gene (MP, ΔNPT, or CP), expressed the CP or GUS genes despite their 3′-proximal localization. The capacity of crTMV IRESCPfor mediating internal translation distinguishes this CP tobamovirus from the well-known-type member of the genus, TMV UI. The equivalent 148-nt sequence from TMV RNA was incapable of mediating internal translation. Two mutants were used to study structural elements of IRESCP. It was concluded that integrity of IRESCPwas essential for internal initiation. The crTMV provides a new example of internal initiation of translation, which is markedly distinct from IRESs shown for picornaviruses and other viral and eukaryotic mRNAs.