Functional in vivo analysis of RNA editing factors in bryophytes and angiosperms

Abstract

RNA editing is an essential posttranscriptional process in the organelles of all land plants, with the exception of a liverwort belonging to the Marchantiidae. The predominant process is the replacement of cytosine residues by uracils in the RNA. Cis-elements located in the 5’ region of target cytidines specify respective RNA editing sites and are recognized by trans-factors. Those trans-factors are members of the PLS class of PPR proteins. Their DYW subclass gained particular interest, as the DYW domain is assumed to harbour the required enzymatic activity.

The liverwort Marchantia polymorpha was transformed with combinations of heterologous chimera PPR proteins to introduce a de novo RNA editing event in the organism that possesses no native RNA editing. Introduction of a single PPR protein, which targets a site in Arabidopsis thaliana, was not sufficient to establish RNA editing. This is explicable by the observation that PPR RNA editing factors from complex systems seem to miss cofactors required for full functionality of the editosome in a bryophyte environment. This assumption was supported by the results of cross-species experiments. A knock-out line of an RNA editing factor in A. thaliana, CWM1, could be functionally complemented by the transformation of a heterologous RNA editing factor from the moss P. patens. The factor from the presumably simple and straightforward RNA editing system with only DYW subclass PPR proteins acting as specificity factors, was able to restore RNA editing defects, at least in part. Vice versa, it was not possible to substitute the corresponding moss PPR protein with the PPR factor from the angiosperm. This supports the hypothesis that PPR proteins from the complex system in angiosperms require additional co-factors which are missing in the simple bryophyte system.

To understand the complexity of RNA editosomes, two more RNA editing factors were analysed. When investigating the RNA editing defects of the PPR protein mutant mef37, a target site of the RNA editing factor MEF19 was also affected. One of the mef37-defective sites is located inside the putative binding sequence for MEF19 and seems to prevent MEF19 from binding. This finding demonstrates how PPR proteins can influence each other.

The knock-out of the editing factor MEF45 results in embryo lethality. The assessment of the binding conditions of the PPR proteins MEF45, MEF37 and SLO4 underlines the importance of a stable binding of the C-terminal part of the PPR domains.