Group II (gII) introns are mobile retroelements that can spread to new DNA sites through retrotransposition, which can be influenced by a variety of host factors. To determine if these host factors bear any relationship to the genomic location of gII introns, we developed a bioinformatic pipeline wherein we focused on the genomic neighborhoods of bacterial gII introns within their native contexts and sought to determine global relationships between introns and their surrounding genes. We found that, although gII introns inhabit diverse regions, these neighborhoods are often functionally enriched for genes that could promote gII intron retention or proliferation. On one hand, we observe that gII introns are frequently found hiding in mobile elements or after transcription terminators. On the other hand, gII introns are enriched in locations in which they could hijack host functions for their movement, potentially timing expression of the intron with genes that produce favorable conditions for retrotransposition. Thus, we propose that gII intron distributions have been shaped by relationships with their surrounding genomic neighbors.
Splicing is a highly regulated process that depends on numerous factors. It is particularly challenging to quantitatively predict how a mutation will affect precursor messenger RNA (mRNA) structure and the subsequent functional consequences. Here we use a novel Mutational Profiling (-MaP) methodology to obtain highly reproducible endogenous precursor and mature mRNA structural probing data in vivo. We use these data to estimate Boltzmann suboptimal ensembles, and predict the structural consequences of mutations on precursor mRNA structure. Together with a structural analysis of recent cryo-EM spliceosome structures at different stages of the splicing cycle, we determined that the footprint of the Bact complex on precursor mRNA is best able to predict splicing outcomes for exon 10 inclusion of the alternatively spliced MAPT gene. However, structure alone only achieves 74% accuracy. We therefore developed a β-regression weighting framework that incorporates splice site strength, structure and exonic/intronic splicing regulatory elements which together achieves 90% accuracy for 47 known and six newly discovered splice-altering variants. This combined experimental/computational framework represents a path forward for accurate prediction of splicing related disease-causing variants.
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.
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