Top3β is an RNA topoisomerase that works with fragile X syndrome protein to promote synapse formation

Topoisomerases are thought to work in DNA, but not RNA, metabolism. Here Xu et al . describe that Top3A can also act as an RNA topoisomerase and find it in a complex with the fragile X syndrome protein FMRP and a binding partner of FMRP, Tudor domain–containing protein 3 (TDRD3), which together promote synapse formation in flies and mice. These data suggest that topoisomerases are needed for both DNA and RNA metabolism.