Albany 2013: Book of Abstracts
June 11-15 2013
©Adenine Press (2012)
Why No Group II Introns in Nuclear Genomes? Translational Repression, RNA-RNA Interactions and Mis-compartmentalization of mRNAs
Group II introns are commonly believed to be the progenitors of spliceosomal introns. However, the notable absence of group II introns from nuclear genomes begs examination. We have shown that although nuclear expression of a group II intron-containing pre-mRNA in yeast results in efficient transcription and protein-dependent splicing, RNA transcripts suffer nonsense-mediated decay (NMD) and translational repression (Chalamcharla et al., 2010). To demonstrate the possible mechanism for the translational repression, we have investigated cellular dynamics of group II intron-containing RNAs, from transcription to cellular localization, by utilizing multi-disciplinary techniques. Specialized 5’-RACE indicates that group II intron-containing RNA and its spliced product are both 5’-capped and 3’-polyadenylated as are most eukaryotic RNA polymerase II transcripts. Additionally, primer extension assay showed that RNA transcription and splicing both initiate from the correct sites. Importantly, molecular pull-down experiments reveal that spliced RNA and its precursor in cell lysates can be co-isolated by affinity resins specialized for the precursor, suggesting interactions between RNAs or RNA-protein complexes. Finally, fluorescence microscopy and immuno-precipitation analyses both demonstrate that the unspliced precursor and spliced RNA are significantly co-localized with processing bodies and stress granules, the two main cellular compartments known to retain untranslated or aberrant RNAs in eukaryotic cells. Consistent with these observations, 5’-3’ truncated intron-containing RNA species were detected by primer extension. We present a model whereby mRNA-pre-mRNA association and mis-compartmentalization of mRNPs result in translational repression of mRNAs that contained group II introns, and that may have resulted in expunging of such introns from nuclear genomes.
This research has been supported by NIH
V. R. Chalamcharla, M. J. Curcio, and M. Belfort. (2010). Nuclear expression of a group II intron is consistent with spliceosomal intron ancestry. Genes & Devel. 24, 827–836