The Chloroplast Trans-Splicing RNA–Protein Supercomplex from the Green Alga Chlamydomonas reinhardtii

In eukaryotes, RNA trans-splicing is a significant RNA modification process for the end-to-end ligation of exons from separately transcribed primary transcripts to generate mature mRNA. So far, three different categories of RNA trans-splicing have been found in organisms within a diverse range. Here, we review trans-splicing of discontinuous group II introns, which occurs in chloroplasts and mitochondria of lower eukaryotes and plants. We discuss the origin of intronic sequences and the evolutionary relationship between chloroplast ribonucleoprotein complexes and the nuclear spliceosome. Finally, we focus on the ribonucleoprotein supercomplex involved in trans-splicing of chloroplast group II introns from the green alga Chlamydomonas reinhardtii. This complex has been well characterized genetically and biochemically, resulting in a detailed picture of the chloroplast ribonucleoprotein supercomplex. This information contributes substantially to our understanding of the function of RNA-processing machineries and might provide a blueprint for other splicing complexes involved in trans- as well as cis-splicing of organellar intron RNAs.

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Peer Review #2 of "Proliferation of group II introns in the chloroplast genome of the green alga Oedocladium carolinianum (Chlorophyceae) (v0.1)"

10.7287/peerj.2627v0.1/reviews/2 ◽

2016 ◽

Keyword(s):

Chloroplast Genome ◽

Peer Review ◽

Green Alga ◽

Group Ii Introns ◽

Group Ii

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Group II Introns Generate Functional Chimeric Relaxase Enzymes with Modified Specificities through Exon Shuffling at Both the RNA and DNA Level

Molecular Biology and Evolution ◽

10.1093/molbev/msaa275 ◽

2020 ◽

Author(s):

Félix LaRoche-Johnston ◽

Rafia Bosan ◽

Benoit Cousineau

Keyword(s):

Genetic Diversity ◽

Insertion Site ◽

Long Terminal Repeat Retrotransposons ◽

Group Ii Introns ◽

Gain Of Function ◽

Intron Insertion ◽

Spliceosomal Introns ◽

Biologically Relevant ◽

Trans Splicing ◽

Group Ii

Abstract Group II introns are large self-splicing RNA enzymes with a broad but somewhat irregular phylogenetic distribution. These ancient retromobile elements are the proposed ancestors of approximately half the human genome, including the abundant spliceosomal introns and non-long terminal repeat retrotransposons. In contrast to their eukaryotic derivatives, bacterial group II introns have largely been considered as harmful selfish mobile retroelements that parasitize the genome of their host. As a challenge to this view, we recently uncovered a new intergenic trans-splicing pathway that generates an assortment of mRNA chimeras. The ability of group II introns to combine disparate mRNA fragments was proposed to increase the genetic diversity of the bacterial host by shuffling coding sequences. Here, we show that the Ll.LtrB and Ef.PcfG group II introns from Lactococcus lactis and Enterococcus faecalis respectively can both use the intergenic trans-splicing pathway to catalyze the formation of chimeric relaxase mRNAs and functional proteins. We demonstrated that some of these compound relaxase enzymes yield gain-of-function phenotypes, being significantly more efficient than their precursor wild-type enzymes at supporting bacterial conjugation. We also found that relaxase enzymes with shuffled functional domains are produced in biologically relevant settings under natural expression levels. Finally, we uncovered examples of lactococcal chimeric relaxase genes with junctions exactly at the intron insertion site. Overall, our work demonstrates that the genetic diversity generated by group II introns, at the RNA level by intergenic trans-splicing and at the DNA level by recombination, can yield new functional enzymes with shuffled exons, which can lead to gain-of-function phenotypes.

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