On the Possibility of an Early Evolutionary Origin for the Spliced Leader Trans-Splicing
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Trans-splicing is a process by which 5′- and 3′-ends of two pre-RNA molecules transcribed from different sites of the genome can be joined together to form a single RNA molecule. The spliced leader (SL) trans-splicing is mediated by the spliceosome and it allows the replacement of 5′-end of pre-mRNA by 5′(SL)-end of SL-RNA. This form of splicing has been observed in many phylogenetically unrelated eukaryotes. Either the SL trans-splicing (SLTS) originated in the last eukaryotic common ancestor (LECA) (or even earlier) and it was lost in most eukaryotic lineages, or this mechanism of RNA processing evolved several times independently in various unrelated eukaryotic taxa. The bioinformatic comparisons of SL-RNAs from various eukaryotic taxonomic groups have revealed the similarities of secondary structures of most SL-RNAs and a relative conservation of their splice sites (SSs) and Sm-binding sites (SmBSs). We propose that such structural and functional similarities of SL-RNAs are unlikely to have evolved repeatedly many times. Hence, we favor the scenario of an early evolutionary origin for the SLTS and multiple losses of SL-RNAs in various eukaryotic lineages.
KeywordsIntron RNA secondary structure Sm-binding site SL-RNA Spliceosome
The last eukaryotic common ancestor
The last universal common ancestor
“Smith antigen” proteins which bind to small nuclear RNAs (snRNAs), i.e., spliceosomal U (U-rich) or SL snRNAs
Capped spliced leader sequence which is transferred to 5′-end of pre-mRNA during SL trans-splicing
Spliced leader (SL) snRNA which serves as donor of capped SL sequences for pre-mRNA
Spliced leader trans-splicing
This work was supported by the Scientific Grant Agency of the Slovak Ministry of Education and the Academy of Sciences (Grants VEGA 1/0626/13 and 1/0535/17), and by project ITMS 26210120024 supported by the Research & Development Operational Programme funded by the ERDF.
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