Project description:Type II topoisomerases (topos) are a ubiquitous and essential class of enzymes that form transient enzyme-bound double-stranded breaks on DNA called cleavage complexes. The location and frequency of these cleavage complexes on DNA is important for cellular function, genomic stability, and a number of clinically important anticancer and antibacterial drugs, e.g., quinolones. We developed a simple high-accuracy end-sequencing (SHAN-seq) method to sensitively map type II topo cleavage complexes on DNA in vitro. Using SHAN-seq, we detected Escherichia coli gyrase and topoisomerase IV cleavage complexes at hundreds of sites on supercoiled pBR322 DNA, approximately one site every ten bp, with frequencies that varied by two-to-three orders of magnitude. These sites included previously identified sites and 20-50 fold more new sites. We show that the location and frequency of cleavage complexes at these sites are enzyme-specific and vary substantially in the presence of the quinolone, ciprofloxacin, but not with DNA supercoil chirality, i.e., negative vs. positive supercoiling. SHAN-seq’s exquisite sensitivity provides an unprecedented single-nucleotide resolution view of the distribution of gyrase and topoisomerase IV cleavage complexes on DNA. Moreover, the discovery that these enzymes can cleave DNA at orders of magnitude more sites than the relatively few previously known sites resolves the apparent paradox of how these enzymes resolve topological problems throughout the genome.
Project description:Messenger RNAs are regulated by a variety of degradation mechanisms in mammalian cells. In the canonical animal microRNA pathway, microRNAs in complex with Argonaute proteins bind to many mRNA targets with imperfect complementarity, leading to degradation of the mRNA through the regular decay machinery. The ancestral “slicer” endonuclease activity of Argonaute2 itself, which requires more extensive complementarity with the target RNA, is not used in this pathway, and has only been observed in two microRNA-guided cases. Nevertheless, the cleavage capacity of mammalian Ago2 is conserved and essential for viability. Here, we assess the endonucleolytic function of Ago2 and other nucleases by identifying cleavage products retaining 5`-phosphate groups in mouse ES cells on a transcriptome-wide scale. We detect a significant signature of Ago2-dependent cleavage events and validate several targets. Unexpectedly, a broader class of Ago2-independent cleavage sites is also observed, indicating participation of additional nucleases in this mode of mRNA regulation. Within this class, we identify a cohort of Drosha-dependent mRNA cleavage events, including one in the Dgcr8 mRNA, that functionally regulate mRNA levels in mES cells. Together, these results highlight the underappreciated role of endonucleolytic cleavage in controlling mRNA fates in mammals. Global 5`-phosphate-dependent RACE in WT, Ago2-KO and Drosha-excised mouse ES cells and human 293S cells
Project description:Messenger RNAs are regulated by a variety of degradation mechanisms in mammalian cells. In the canonical animal microRNA pathway, microRNAs in complex with Argonaute proteins bind to many mRNA targets with imperfect complementarity, leading to degradation of the mRNA through the regular decay machinery. The ancestral “slicer” endonuclease activity of Argonaute2 itself, which requires more extensive complementarity with the target RNA, is not used in this pathway, and has only been observed in two microRNA-guided cases. Nevertheless, the cleavage capacity of mammalian Ago2 is conserved and essential for viability. Here, we assess the endonucleolytic function of Ago2 and other nucleases by identifying cleavage products retaining 5`-phosphate groups in mouse ES cells on a transcriptome-wide scale. We detect a significant signature of Ago2-dependent cleavage events and validate several targets. Unexpectedly, a broader class of Ago2-independent cleavage sites is also observed, indicating participation of additional nucleases in this mode of mRNA regulation. Within this class, we identify a cohort of Drosha-dependent mRNA cleavage events, including one in the Dgcr8 mRNA, that functionally regulate mRNA levels in mES cells. Together, these results highlight the underappreciated role of endonucleolytic cleavage in controlling mRNA fates in mammals.