Trans-splicing

Unscrambling genetic information at the RNA level

Genomics aims at unraveling the blueprint of life; however, DNA sequence alone does not always reveal the proteins and structural RNAs encoded by the genome. The reason is that genetic information is often encrypted. Recognizing the logic of encryption, and understanding how living cells decode hidden information--at the level of DNA, RNA or protein--is challenging. RNA-level decryption includes topical RNA editing and more 'macroscopic' transcript rearrangements. The latter events involve the four types of introns recognized to date, notably spliceosomal, group I, group II, and archaeal/tRNA splicing. Intricate variants, such as alternative splicing and trans-splicing, have been reported for each intron type, but the biological significance has not always been confirmed. Novel RNA-level unscrambling processes were recently discovered in mitochondria of dinoflagellates and diplonemids, and potentially euglenids. These processes seem not to rely on known introns, and the corresponding molecular mechanisms remain to be elucidated.

Split genes: another surprise from Giardia

Some genes in the candidate early-branching eukaryote Giardia lamblia occur in separate pieces, transcribed from non-contiguous chromosomal locations. The pre-mRNAs from the separate pieces apparently find each other by regions of complementarity and are subsequently spliced together by the spliceosome. Could genes in pieces, transcribed into separate pre-mRNAs, have been an early feature of spliceosomal evolution?