Spliced Leader Trans-splicing: A Conserved Modification Mechanism of Pre-mRNA

In the process of SL(spliced leader) trans-splicing,with the help of spliceosome,SL as a mini-exon is added to the 5'-end of pre-mRNA to mature mRNA through the sites SD in SL RNA and SA in pre-mRNA.This modification pattern of pre-mRNA is completely retained in evolutionarily conservative molecules.Sometimes,the size of SL from different organisms is greatly different,and,in some creature,there are two different SLs that play different roles in the process of pre-mRNA.Although the nucleotide constituent of different SL RNAs varies significantly,their structures show high similarity,such as a TMG cap at the 5'-terminal,conservative sites SD and Sm,the secondary structure of SL RNA.The phenomenon of SL trans-splicing has crucially evolutionary significance.Because of the loss of SA site or the gain of SD site in the transcriptional process of pre-mRNA,the SL trans-splicing way is discarded in evolution in most vertebrates.The lack may be one of the reasons that give rise to the increase of C value.