Topoisomerase 3β Interacts with RNAi Machinery to Promote Heterochromatin Formation and Transcriptional Silencing in Drosophila [RNA-Seq]

Topoisomerases solve topological problems during DNA metabolism, but whether they participate in RNA metabolism remains unclear. Top3b represents a family of topoisomerases carrying activities for both DNA and RNA. Here we show that in Drosophila, Top3b interacts biochemically and genetically with the RNAi-induced silencing complex (RISC) containing AGO2, p68 RNA helicase, and FMRP. Top3b and RISC mutants are similarly defective in heterochromatin formation and transcriptional silencing by position-effect variegation assay. Moreover, both Top3b and AGO2 mutants exhibit reduced levels of heterochromatin protein HP1 in pericentric heterochromatin. Furthermore, expression of several genes and transposable elements in heterochromatin is increased in the Top3b mutant. Notably, Top3b mutants defective in either RNA binding or catalytic activity are deficient in promoting HP1 recruitment and silencing of transposable elements. Our data suggest that Top3b may act as an RNA topoisomerase in siRNA-guided heterochromatin formation and transcriptional silencing. We examined the RNA expression profile in RNA/DNA topoisomerase (Top3b) mutant flies as compared to w1118 control flies. We isolated the head of each fly and extracted the total RNA for RNA-seq.