3 years ago

Processing of Potato Spindle Tuber Viroid (PSTVd) RNAs in Yeast, a Nonconventional Host.

Mukkara, Owens, Bruist, Baumstark, Friday
Potato spindle tuber viroid (PSTVd) is a circular, single-stranded, noncoding RNA plant pathogen that is a useful model for the processing of noncoding RNA in eukaryotes. Infective PSTVd circles are replicated via an asymmetric rolling circle mechanism to form linear multimeric RNAs. An endonuclease cleaves these into monomers; a ligase seals these into mature circles. All eukaryotes may have such enzymes for processing noncoding RNA. As a test, we investigated the processing of three PSTVd RNA constructs in the yeast Saccharomyces cerevisiae. Of these, only one form, the construct that adopts a previously described tetraloop-containing conformation (TL), produces circles.TL has 16 nucleotides of the 3' end duplicated at the 5' end and a 3' end produced by self-cleavage of a delta ribozyme. The other two constructs, an exact monomer flanked by ribozymes, and a trihelix-forming RNA with requisite 5' and 3' duplications do not produce circles. The TL circles contain non-native nucleotides resulting from the 3' -end created by the ribozyme and the 5' -end from an endolytic cleavage by yeast at a site distinct from where potato enzymes cut these RNAs. RNAs from all three transcripts are cleaved in places not on path for circle formation, and are likely RNA decay. We propose that these constructs fold into distinct RNA structures that interact differently with host cell RNA metabolism enzymes, resulting in varying susceptibility to degradation versus processing. We conclude that PSTVd RNA is opportunistic and may use different processing pathways in different hosts.IMPORTANCE In higher eukaryotes, the majority of transcribed RNA does not encode proteins. These noncoding RNA are responsible for messenger RNA regulation, control of the expression of regulatory microRNAs, sensing changes in the environment using riboswitches (RNAs that change shape in response to environmental signals), catalysis, and more roles that are still being uncovered. Some of these functions may be remnants from the RNA world, and as such would be part of the evolutionary past of all forms of modern life. Viroids are noncoding RNAs that can cause disease in plants. Since they encode no proteins, they depend on their own RNA and host proteins for replication and pathogenicity. It is likely that viroids hijack critical host RNA pathways for processing the host's own noncoding RNA. These pathways are still unknown. Elucidating these pathways should reveal new biological functions of noncoding RNA.

Publisher URL: http://doi.org/10.1128/JVI.01078-17

DOI: 10.1128/JVI.01078-17

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