Mobile group II intron : host factors, directed evolution, and gene targeting in human cells
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Mobile group II introns are retroelements that are found in prokaryotes, archaea, and the organelles of plants and fungi, but not in the nuclear genomes of eukaryotes. They consist of a catalytically active RNA and intron-encoded reverse transcriptase, which together promote site-specific integration into DNA sites in a mechanism called retrohoming. The group II intron Ll.LtrB has been developed into a programmable, DNA-targeting agent called "targetron", which is widely used in bacteria and an attractive technology for gene targeting in eukaryotes. However, group II intron genome targeting in human cells has not been equivocally shown. This dissertation focuses on the hypothesis that the low Mg2+-concentrations found in higher eukaryotes present a natural barrier to group II introns. First, I studied E. coli host proteins that aid group II intron retrohoming and found that synthesis of a second DNA-strand relies on host replication restart proteins. Next, I demonstrated that mutations in the distal stem of the catalytic core domain V (DV) improve Ll.LtrB retrohoming in a low Mg2+-concentration E. coli mutant and in biochemical assays. These results suggest that DV is involved in an RNA-folding step that becomes rate limiting at low Mg2+. Subsequently, I performed directed evolution of the intron RNA by injecting in vitro prepared mutant intron libraries into Xenopus laevis oocyte nuclei. The mutations were analyzed using Roche 454 sequencing to generate an intron fitness landscape, which revealed conserved positions and potentially beneficial mutations, enabling enhanced retrohoming in Xenopus oocytes. Finally, I used a hybrid Pol II/T7 Ll.LtrB eukaryotic expression system to show that high exogenous MgCl2 in the growth media enables retrohoming into plasmids and genomic DNA in human cells. In vivo directed evolution and mutation analyses using PacBio RS circular consensus sequencing indicated that only a few mutations may improve intron activity in human cells. This dissertation provides evidence that efficient group II intron retrohoming in human cells is limited by low Mg2+-concentrations and develops new approaches for overcoming this limitation to enable use of group II introns for gene targeting in higher organisms.