Abstract
DNA topoisomerase I (TOP1) has an important role in maintaining DNA topology by relaxing supercoiled DNA. Here we show that the K391 and K436 residues of TOP1 are SUMOylated by the PIAS1-SRSF1 E3 ligase complex in the chromatin fraction containing active RNA polymerase II (RNAPIIo). This modification is necessary for the binding of TOP1 to RNAPIIo and for the recruitment of RNA splicing factors to the actively transcribed chromatin, thereby reducing the formation of R-loops that lead to genome instability. RECQ5 helicase promotes TOP1 SUMOylation by facilitating the interaction between PIAS1, SRSF1 and TOP1. Unexpectedly, the topoisomerase activity is compromised by K391/K436 SUMOylation, and this provides the first in vivo evidence that TOP1 activity is negatively regulated at transcriptionally active chromatin to prevent TOP1-induced DNA damage. Therefore, our data provide mechanistic insight into how TOP1 SUMOylation contributes to genome maintenance during transcription.
Highlights
DNA topoisomerase I (TOP1) has an important role in maintaining DNA topology by relaxing supercoiled DNA
We have discovered that the helicase domain of RECQ5 interacts directly with TOP1 and facilitates TOP1 SUMOylation at lysine (K) 391 and 436 residues at highly transcribed loci in a manner that is dependent on the PIAS1 E3 ligase and its cofactor SRSF1
We demonstrated that the TOP1 molecules are SUMOylated at K391 and K436 residues in a transcriptiondependent but DNA damage-independent manner (Figs 1 and 2)
Summary
DNA topoisomerase I (TOP1) has an important role in maintaining DNA topology by relaxing supercoiled DNA. K391/K436 SUMOylation promotes the interaction of TOP1 with RNAPIIo and is necessary for the efficient recruitment of mRNA processing factors to the transcriptionally active chromatin to prevent R-loops This function explains why RECQ5-mutant cells defective in K391/ K436 SUMOylation accumulate R-loops and R-loop-induced DSBs. In addition, we further demonstrate that K391/K436 SUMOylation significantly reduces the catalytic activity of TOP1, indicating that TOP1-dependent relaxation of negatively supercoiled DNA is unlikely to contribute to R-loop prevention at highly transcribed gene loci. We further demonstrate that K391/K436 SUMOylation significantly reduces the catalytic activity of TOP1, indicating that TOP1-dependent relaxation of negatively supercoiled DNA is unlikely to contribute to R-loop prevention at highly transcribed gene loci This negative regulation of TOP1 activity may be necessary to prevent the accumulation of trapped TOP1–DNA complexes at these regions and genome instability[12,17]
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