Evolutionary Dynamics and Polyadenylation Mechanisms of the Tal SINE Family in Talpidae Genomes

Short Interspersed Elements (SINEs) are eukaryotic non-autonomous retrotransposons that rely on RNA polymerase III (pol III) for transcription. A specialized mammalian subset, designated T+ SINEs, harbors a canonical polyadenylation signal (AATAAA), a pol III terminator, and a 3′-terminal A-rich tail. Consequently, these elements undergo AAUAAA-dependent polyadenylation, unusual for pol III transcripts, yielding a poly(A) tail required for the retrotransposition of such SINEs. Here, a comprehensive investigation of Tal SINEs across six talpid species (true moles and desmans) demonstrated their clear affiliation with T+ SINEs. Computational analysis identified 300,000–600,000 Tal copies per genome, classified into five to eight subfamilies of varying evolutionary ages. The Tal family originated in the Talpidae lineage following its divergence from shrew moles (Uropsilinae) approximately 42 million years ago. Analysis of pol III transcription terminators revealed that the moderate TCTTT terminator is most prevalent in young subfamilies. Furthermore, distinct tribes of young, recently integrated Tal copies were identified. Functional assays using a cloned Tal copy of the e_Dmo subfamily, involving the generation of deletion mutants and their transfection into HeLa cells, allowed us to map two internal regions (the β- and τ-signals) required for efficient transcript polyadenylation. The upstream β-signal (CCCACATGG) shares sequence similarity with the mouse B2 SINE β-signal. Consistent with other characterized T+ SINEs, the downstream τ-signal is mediated by extended polypyrimidine-rich tracts. Thus, Tal represents a classic prototypical T+ SINE, and these data provide insights into the conserved organizational blueprints of pol III-transcribed retrotransposons.