Reviving the exosome.

In the nucleus of eukaryotic cells, RNA decay in the 3'-5' direction is carried out by a complex of exonucleases called the exosome. Surprisingly, the purified exosome shows only weak activity in vitro. Two papers in this issue of Cell, by LaCava et al. (2005) and Wyers et al. (2005), and a third report by Vanacova et al. (2005) help to solve this riddle by characterizing a new RNA decay-activating complex containing a poly(A) polymerase. In addition, they identify new unconventional RNA polymerase II transcription units.