A Six-Nucleotide Segment within the 3′ Untranslated Region of Hibiscus Chlorotic Ringspot Virus Plays an Essential Role in Translational Enhancement

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Abstract
RNA plant viruses use various translational regulatory mechanisms to control their gene expression. Translational enhancement of viral mRNAs that leads to higher levels of protein synthesis from specific genes may be essential for the virus to successfully compete for cellular translational machinery. The control elements have yet to be analyzed for members of the genus Carmovirus , a small group of plant viruses with positive-sense RNA genomes. In this study, we examined the 3′ untranslated region (UTR) of hibiscus chlorotic ringspot virus (HCRSV) genomic RNA (gRNA) and subgenomic RNA (sgRNA) for its role in the translational regulation of viral gene expression. The results showed that the 3′ UTR of HCRSV significantly enhanced the translation of several open reading frames on gRNA and sgRNA and a viral gene in a bicistronic construct with an inserted internal ribosome entry site. Through deletion and mutagenesis studies of both the bicistronic construct and full-length gRNA, we demonstrated that a six-nucleotide sequence, GGGCAG, that is complementary to the 3′ region of the 18S rRNA and a minimal length of 180 nucleotides are required for the enhancement of translation induced by the 3′ UTR.