One form of trans-splicing is the 'spliced leader' type, which is primarily found in protozoans (e.g. trypanosomes) and in lower invertebrates such as nematodes. This results in the addition of a capped, noncoding, spliced leader sequence to the 5' end of mRNAs.
Another form of trans-splicing is the 'discontinuous group II intron' type that occurs in plant/algal chloroplasts and plant mitochondria. This results in the joining of two independently transcribed coding sequences. Both spliced-leader and discontinuous group II intron trans-splicing are mechanistically similar to conventional nuclear pre-mRNA cis-splicing. Trans-splicing also occurs in mammalian cells, just as cis-splicing occurs in trypanosomes. It has been suggested that both trans- and cis-splicing are ancient acquisitions of the eukaryotic cell. (Abstract)
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