Analysis of RNA Transcribed by RNA Polymerase III from B2 SINEs in Mouse Cells

<b>Background/Objectives:</b> SINEs (short interspersed elements) are eukaryotic non-autonomous retrotransposons. They are transcribed by RNA polymerase III (pol III) and generate non-coding RNAs. The 3' end of many mammalian SINEs contains a polyadenylation signal (AATAAA), a pol III transcription terminator, and an A-rich tail. Studies have shown that, in human HeLa cells that have been transiently transfected with such SINEs, short pol III-generated SINE transcripts undergo polyadenylation, resulting in the addition of a long poly(A)-tail. Notably, this AAUAAA-dependent polyadenylation is not characteristic of any other transcripts synthesized by pol III. B2 SINEs, found in the genomes of mouse-like rodents, exemplify all these features. <b>Methods:</b> In this study, we implemented a novel approach to sequencing pol III-generated B2 transcripts from mouse cell cultures (L929 and 4T1) and organs (brain and testis). <b>Results:</b> Transcription occurred in 16,000-20,000 B2 copies in each cell type, 51-62% of which were transcribed in all four cell types. Effective transcription terminators (e.g., TCT_>3 and T_≥4) were found in approximately 40% of the transcribed B2 copies. The transcripts of these B2 copies contained a truncated terminator sequence, as pol III transcriptional arrest is known to occur within the terminator, with a poly(A)-tail immediately downstream. Such a tail could only have formed through RNA polyadenylation. <b>Conclusions:</b> These results demonstrate that B2 transcripts synthesized by pol III are capable of polyadenylation in mouse cells. We discuss the transcription of B2 copies with and without moderately efficient pol III terminators (TCTTT) and provide examples of the polyadenylation of such transcripts.