Abstract
The Kaposi's sarcoma-associated herpesvirus (KSHV) protein kinase, encoded by ORF36, functions to phosphorylate cellular and viral targets important in the KSHV lifecycle and to activate the anti-viral prodrug ganciclovir. Unlike the vast majority of mapped KSHV genes, no viral transcript has been identified with ORF36 positioned as the 5′-proximal gene. Here we report that ORF36 is robustly translated as a downstream cistron from the ORF35–37 polycistronic transcript in a cap-dependent manner. We identified two short, upstream open reading frames (uORFs) within the 5′ UTR of the polycistronic mRNA. While both uORFs function as negative regulators of ORF35, unexpectedly, the second allows for the translation of the downstream ORF36 gene by a termination-reinitiation mechanism. Positional conservation of uORFs within a number of related viruses suggests that this may be a common γ-herpesviral adaptation of a host translational regulatory mechanism.
Highlights
Translation initiation of eukaryotic mRNAs is dependent on the 59 mRNA cap and proceeds by ribosomal scanning until recognition of an AUG codon in a favorable context [1,2]
Kaposi’s sarcoma-associated herpesvirus (KSHV) expresses a number of transcripts with the potential to generate multiple proteins, yet relies on the cellular translation machinery that is primed to synthesize only one protein per mRNA
We report that the viral transcript encompassing ORF35–37 is able to direct synthesis of two proteins and that the translational switch is regulated by two short upstream open reading frames in the native 59 untranslated region. uORFs are elements commonly found upstream of mammalian genes that function to interfere with unrestrained ribosomal scanning and repress translation of the major ORF
Summary
Translation initiation of eukaryotic mRNAs is dependent on the 59 mRNA cap and proceeds by ribosomal scanning until recognition of an AUG codon in a favorable context [1,2]. As a consequence of the translation machinery not engaging start codons at internal positions within the mRNA, eukaryotic transcripts generally encode only one functional protein. For the majority of mRNAs the most 59-proximal AUG is selected, strategies exist to bypass upstream start codons to enable downstream initiation. When an upstream AUG is followed shortly thereafter by an in-frame termination codon, ribosomes can reinitiate translation, albeit with reduced efficiency, at a downstream AUG. These upstream open reading frames (uORFs) presumably permit translation of a downstream gene because factors necessary for initiation have not yet dissociated during the short elongation period. Additional, rare, examples of internal ORF translation exist, for example after ribosome shunting over a highly structured upstream sequence [5,6,7,8], or upon direct 40S recruitment via internal ribosome entry sites (IRESs) [9,10,11,12,13]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
