Topoisomerase 3b Facilitates piRNA Biogenesis to Promote Transposon Silencing and Germ Cell Development

Topoisomerases typically function in nucleus to relieve topological stress in DNA. Here we show that a dual-activity topoisomerase Top3b and its partner TDRD3 largely localize in cytoplasm, and interact biochemically and genetically with piRNA processing enzymes to promote piRNA biogenesis, post-transcriptional gene silencing (PTGS) of transposons, and Drosophila germ cell development. Top3b requires its topoisomerase activity to promote PTGS of a transposon reporter, and preferentially silences long and highly-expressed transposons, suggesting that RNAs with these features may produce more topological stress for topoisomerases to solve. The double mutants between Top3b and piRNA processing enzymes exhibit stronger disruption of the signatures and levels of germline piRNAs, more de-silenced transposons, and larger defects in germ cells, than either single mutant. Our data suggest that Top3b can act in an RNA-based process—piRNA biogenesis and PTGS of transposons; and this function is required for Top3b to promote normal germ cell function. Overall design: Test if Top3b mutant exhibits de-silencing of transposons by genetic interaction with piRNA pathway mutants - aub, ago3, vas, armi, zuc. We performed RNA-seq and small RNA-seq to profile transposon level as compared to control.