The mRNA maturation processes determine mRNA degradation, translation efficiency as well as mRNA export, but could be a source of aberrant RNA as well. mRNA decay is an important part of regulation of gene expression. It leads to degradation of aberrant mRNA as well as unwanted mRNA in a certain time and a space to save an energy and recycle carbon. The decay could be triggered by range of signals and can process through various pathways as: deadenylation-dependent, deadenylation-independent, endonuclease-mediated, nonsense-mediated, non-stop, or no-go decay as well as RNA interference. Variety of aberrant RNA can serve as a substrate for the decay pathways in the process known as mRNA surveillance mechanism. Improperly located stop codon can trigger nonsense-mediated decay, and ectopic localized 5’-donor splice site sequence (5’ss) may inhibit polyadenylation and down-regulate gene expression, and double stranded RNA can be recognized by RNA interference machinery. Moreover, dysfunctional pathway could be substitute by others leading to removal of redundant mRNA.

Chimeric constructs were engineered to harbor a tandem insertion of 5’ss in the 3’UTR of GFP (GFP-2x5’ss). Their transient expression in Nicotiana benthamiana caused a down-regulation of GFP expression. This has previously been shown with a Renilla luciferase construct in animal cells (P. Fortes 2003). Authors suggested that such modification leads to inhibition of polyadenylation of mRNA resulting in the retention of RNA at or near the transcription site, and subsequent degradation by an exosome. We also have shown an inhibitory effect of 2x5’ss on polyadenylation. We found that this down-regulation is not only directed against modified mRNA itself but is extended to co-expressed mRNA sharing sequence homology. Down-regulation was observed both at the protein and mRNA level. Observed production of siRNA imply a link between down-regulatory mechanisms triggered by 2x5’ss and the RNAi pathway. This suggests that at least two distinct steps are operating in this process: polyadenylation inhibition and mRNA degradation through RNAi pathway.

Our data suggests that incorrectly placed 5’ss – pre-mRNA maturation signal – can serve as an RNA quality marker and could be utilized by mRNA surveillance mechanisms and subsequently mRNA targeted for degradation through the RNAi pathway.

P.A.Furth et al. Mol.Cell Biol (1994), 14:5278,

P.Fortes et al. PNAS USA (2003), 100:8264

This work was supported by EU MEIF-CT-2005-515334 to KW

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