Distinct and redundant roles of nuclear RNA exosome targeting complexes

Degradation of transcripts in mammalian nuclei is primarily facilitated by the RNA exosome. To obtain substrate specificity, the exosome is aided by adaptors; in the nucleoplasm, the Nuclear EXosome Targeting (NEXT) complex and the PolyA (pA) eXsome Targeting (PAXT) connection. However, how exact targeting is achieved remains enigmatic. Employing high-resolution 3'end sequencing of both steady state and newly produced pA+ and pA- RNA, we demonstrate that NEXT substrates arise from heterogenous and predominantly pA- 3'ends often covering kb-wide genomic regions. In contrast, PAXT targets harbor well-defined pA+ 3'ends defined by canonical pA site usage. Irrespective this clear division, NEXT and PAXT act redundantly in two ways: i) Regional redundancy: The majority of exosome-targeted transcription units produce both NEXT- and PAXT-sensitive RNA isoforms; and ii) Isoform redundancy: The PAXT connection ensures the fail-safe decay of post-transcriptionally polyadenylated NEXT targets. In conjunction, this provides for the efficient nuclear removal of superfluous RNA. Overall design: RNA 3'end seq of total and early (10min 4sU labelled) pA+ and pA+ + pA- RNA 3' ends after knock-down of exosome co-factors in HeLa cells .