Determinants of SINE mobilization in the domestic dog.

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Abstract Transposable elements (TEs) have abundantly colonized eukaryotic genomes and are profound mediators of genomic variation, phenotypes, and disease. In humans and other mammals, the vast majority of TE families are defective due to accumulated mutations within the host, but some remain capable of generating new copies. Intracellular spread of such elements can lead to sporadic disease and variation via insertional mutagenesis or by facilitating rearrangements through non-allelic homologous recombination. Understanding the mutagenic potential offered from a particular TE family requires refinement of its lineages, individual copies, and functional properties that affect mobilization capacities within the host. The short and long interspersed elements (SINEs and LINEs) mobilize through an intracellular ‘copy and paste’ process termed retrotransposition. SINEs are non- autonomous and mobilize by recruiting functions in trans from a retrotransposition-competent LINE, accomplished in part by L1 ORF2 protein (ORF2p) poly(A)-tail recognition on the RNA intermediate. The human SINE Alu originated ~65 mya from the 7SL RNA of signal recognition particle (SRP), a cytoplasmic ribonucleoprotein that docks on the ribosome during co-translational targeting to the secretory pathway. The derived Alu exists as two 7SL ‘Alu’ monomers in tandem; both retain 7SL ancestral structure and cytoplasmic binding partners SRP9/14 that direct Alu RNA to ribosome, where it then diverts L1 ORF2p for its own encoding mRNA. Distinct in their origin, SINEs of tRNA ancestry include young families of conserved structure that are highly mobilized in some species. These observations and our own findings lead us to hypothesize that mobilization of the tRNA SINE intermediate is achieved by association with evolutionarily conserved cellular factors in a similar but biologically distinct model to that of Alu. We previously examined a mobile ~180bp SINE of tRNALys ancestry from the canine, SINCEC_Cf, that exhibits high levels of polymorphism, sequence conservation, and insertions linked to disease and artificially selected phenotypes. We have demonstrated bona fide L1Cf-driven retrotransposition of its own mRNA in cis and a synthetic SINCEC_Cf consensus in trans at rates considered ‘hot’ in comparison to those observed for Alu/L1Hs. Moreover, mobilization assays of naturally occurring SINCEC_Cf variants expose alterations to the consensus primary sequence that directly affect mobilization. We take advantage of this model to address our hypothesis and identify structural factors required for mobilization of the tRNA SINE using a combination of genomic, biochemical, molecular, and genetic experimental approaches. This proposal will provide the foundation for SINE mobilization properties in the canine model and a variant resource for comparative genomic analysis in application to human health.