Group II intron and repeat-rich red algal mitochondrial genomes demonstrate the dynamic recent history of autocatalytic RNAs

Background Group II introns are mobile genetic elements that can insert at specific target sequences, however, their origins are often challenging to reconstruct because of rapid sequence decay following invasion and spread into different sites. To advance understanding of group II intron spread, we studied the intron-rich mitochondrial genome (mitogenome) in the unicellular red alga, Porphyridium. Results Analysis of mitogenomes in three closely related species in this genus revealed they were 3–6-fold larger in size (56–132 kbp) than in other red algae, that have genomes of size 21–43 kbp. This discrepancy is explained by two factors, group II intron invasion and expansion of repeated sequences in large intergenic regions. Phylogenetic analysis demonstrates that many mitogenome group II intron families are specific to Porphyridium, whereas others are closely related to sequences in fungi and in the red alga-derived plastids of stramenopiles. Network analysis of intron-encoded proteins (IEPs) shows a clear link between plastid and mitochondrial IEPs in distantly related species, with both groups associated with prokaryotic sequences. Conclusion Our analysis of group II introns in Porphyridium mitogenomes demonstrates the dynamic nature of group II intron evolution, strongly supports the lateral movement of group II introns among diverse eukaryotes, and reveals their ability to proliferate, once integrated in mitochondrial DNA.

Structural basis for template switching by a group II intron-encoded non-LTR-retroelement reverse transcriptase

Reverse transcriptases (RTs) can template switch during cDNA synthesis, enabling them to join discontinuous nucleic acid sequences. Template switching plays crucial roles in retroviral replication and recombination, is used for adapter addition in RNA-seq, and may contribute to retroelement fitness by enabling continuous cDNA synthesis on damaged templates. Here, we determined an X-ray crystal structure of a template-switching complex of a group II intron RT bound simultaneously to an acceptor RNA and donor RNA template/DNA heteroduplex with a 1-nt 3'-DNA overhang. The latter mimics a completed cDNA after non-templated addition (NTA) of a nucleotide complementary to the 3' nucleotide of the acceptor as required for efficient template switching. The structure showed that the 3' end of the acceptor RNA binds in a pocket formed by an N-terminal extension (NTE) present in non-long-terminal-repeat (LTR)-retroelement RTs and the RT fingertips loop, with the 3' nucleotide of the acceptor base paired to the 1-nt 3'-DNA overhang and its penultimate nucleotide base paired to the incoming dNTP at the RT active site. Analysis of structure-guided mutations identified amino acids that contribute to acceptor RNA binding and a phenylalanine near the RT active site that mediates NTA. Mutation of the latter residue decreased multiple sequential template switches in RNA-seq. Our results provide new insights into the mechanisms of template switching and NTA by RTs, suggest how these reactions could be improved for RNA-seq, and reveal common structural features for template switching by non-LTR-retroelement RTs and viral RNA-dependent RNA polymerases.

Low-bias ncRNA Libraries using Ordered Two-Template Relay: Serial Template Jumping by a Modified Retroelement Reverse Transcriptase

Non-long terminal repeat (non-LTR) and group II intron retroelements encode reverse transcriptases (RTs) that copy the retroelement transcript directly into host cell DNA, often at specific target sites. Biochemical characterization of these enzymes has been limited by recombinant expression and purification challenges, hampering understanding of their transposition mechanism and their exploitation for research and biotechnology. Properties of retroelement RTs substantiate their application for end-to-end RNA sequence capture. To investigate this utility, we first compared a non-LTR RT from Bombyx mori and a group II intron RT from Eubacterium rectale. Only the non-LTR RT showed processive template jumping, producing one cDNA from discontinuous templates each copied end-to-end. We also discovered an unexpected terminal deoxynucleotidyl transferase activity of the RTs that adds nucleotide(s) of choice to 3’ ends of single-stranded RNA or DNA. Combining these two types of activity with additional insights about non-templated nucleotide additions to duplexed cDNA product, we developed a streamlined protocol for linking Next Generation Sequencing (NGS) adaptors to both cDNA ends in a single RT reaction. When benchmarked using a reference pool of microRNAs (miRNAs), library production using modified non-LTR retroelment RT for Ordered Two-Template Relay (OTTR) outperformed all commercially available kits and rivaled the low bias of technically demanding home-brew protocols. We applied OTTR to inventory RNAs purified from extracellular vesicles (EVs), identifying miRNAs as well as myriad other non-coding (nc) RNAs and ncRNA fragments. Our results establish the utility of OTTR for automation-friendly, low-bias, end-to-end RNA sequence inventories of complex ncRNA samples.SignificanceRetrotransposons are non-infectious mobile genetic elements that proliferate in host genomes via an RNA intermediate that is copied into DNA by a reverse transcriptase (RT) enzyme. RTs are important for biotechnological applications involving information capture from RNA, since RNA is first converted into complementary DNA for detection or sequencing. Here, we biochemically characterize RTs from two retroelements and uncover several activities that allowed us to design a streamlined, efficient workflow for determining the inventory of RNA sequences in processed RNA pools. The unique properties of non-retroviral RT activities obviate many technical issues associated with current methods of RNA sequence analysis, with wide applications in research, biotechnology, and diagnostics.

Methylation of rRNA as a host defense against rampant group II intron retrotransposition

Background Group II introns are mobile retroelements, capable of invading new sites in DNA. They are self-splicing ribozymes that complex with an intron-encoded protein to form a ribonucleoprotein that targets DNA after splicing. These molecules can invade DNA site-specifically, through a process known as retrohoming, or can invade ectopic sites through retrotransposition. Retrotransposition, in particular, can be strongly influenced by both environmental and cellular factors. Results To investigate host factors that influence retrotransposition, we performed random insertional mutagenesis using the ISS1 transposon to generate a library of over 1000 mutants in Lactococcus lactis, the native host of the Ll.LtrB group II intron. By screening this library, we identified 92 mutants with increased retrotransposition frequencies (RTP-ups). We found that mutations in amino acid transport and metabolism tended to have increased retrotransposition frequencies. We further explored a subset of these RTP-up mutants, the most striking of which is a mutant in the ribosomal RNA methyltransferase rlmH, which exhibited a reproducible 20-fold increase in retrotransposition frequency. In vitro and in vivo experiments revealed that ribosomes in the rlmH mutant were defective in the m3Ψ modification and exhibited reduced binding to the intron RNA. Conclusions Taken together, our results reinforce the importance of the native host organism in regulating group II intron retrotransposition. In particular, the evidence from the rlmH mutant suggests a role for ribosome modification in limiting rampant retrotransposition.

Exon and protein positioning in a pre-catalytic group II intron RNP primed for splicing

Group II introns are the putative progenitors of nuclear spliceosomal introns and use the same two-step splicing pathway. In the cell, the intron RNA forms a ribonucleoprotein (RNP) complex with the intron-encoded protein (IEP), which is essential for splicing. Although structures of spliced group II intron RNAs and RNP complexes have been characterized, structural insights into the splicing process remain enigmatic due to lack of pre-catalytic structural models. Here, we report two cryo-EM structures of endogenously produced group II intron RNPs trapped in their pre-catalytic state. Comparison of the catalytically activated precursor RNP to its previously reported spliced counterpart allowed identification of key structural rearrangements accompanying splicing, including a remodeled active site and engagement of the exons. Importantly, altered RNA–protein interactions were observed upon splicing among the RNP complexes. Furthermore, analysis of the catalytically inert precursor RNP demonstrated the structural impact of the formation of the active site on RNP architecture. Taken together, our results not only fill a gap in understanding the structural basis of IEP-assisted group II intron splicing, but also provide parallels to evolutionarily related spliceosomal splicing.