Key result
Group II introns Ll.LtrB and Ef.PcfG catalyze the formation of functional chimeric relaxase enzymes through exon shuffling, yielding gain-of-function phenotypes that support bacterial conjugation.
Why the study?
Bacterial group II introns were largely considered harmful selfish retroelements, but whether their intergenic trans-splicing pathway generates functional diversity remained to be clarified.
Group II introns can generate functional chimeric enzymes with modified specificities through exon shuffling, leading to gain-of-function phenotypes in bacteria.
Answers are grounded in this paper and cite their sources.
Reveals functional exon shuffling in bacteria; leaves open relevance to microbial adaptation or resistance.
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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LaRoche-Johnston et al. (2020) studied this question. Group II Introns (Ll.LtrB and Ef.PcfG) vs. Wild-type relaxase enzymes and branchpoint mutants was evaluated on Conjugation efficiency and chimeric mRNA/protein production. Group II introns Ll.LtrB and Ef.PcfG catalyze the formation of functional chimeric relaxase enzymes through exon shuffling, yielding gain-of-function phenotypes that support bacterial conjugation.
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