Mitter M, Gasser C, Takacs Z, Langer CCH, Tang W, Jessberger G, Beales CT, Neuner E, Ameres SL, Peters JM, Goloborodko A, Micura R, Gerlich DW.

Nature. 2020 Oct;586(7827):139-144. doi:10.1038/s41586-020-2744-4

Conformation of sister chromatids in the replicated human genome.

Sister-chromatid-sensitive Hi-C (scsHi-C) “4-thio-thymidine (4sT) converted into 5mC by OsO4/NH4Cl” “A read was assigned to the Watson strand if it contained two or more A-to-G mutations and no T-to-C mutations. Similarly, if a read contained two or more T-to-C mutations, but no A-to-G mutations it was assigned to the Crick strand. Then, contacts were classified as cis sister contacts if (after correcting for the opposite read-strandedness of Illumina sequencing of the two mates) both mates mapped to the same strand. Conversely, contacts were classified as trans sister contacts if the two mates mapped to opposing strands.”“Highly paired TADs were markedly enriched in trimethylation of lysine 27 of histone 3 (H3K27me3).” “Trans sister contacts were particularly enriched at many TAD boundaries.“ The cohesin loading factor NIPBL was homozygously tagged with auxin-inducible degrons to deplete loop-forming cohesin. “Loop-forming cohesin is necessary to separate sister chromatids within TADs, resulting in locally enriched sister-chromatid contacts at TAD boundaries.” Sororin was homozygously tagged with AID to deplete the pool of cohesin that mediates sister-chromatid cohesion. “The sororin-stabilized pool of cohesin is [...] not required to form intra-chromatid loops or TADs in G2, but it is required to prevent the separation of sister chromatids and to maintain their global alignment during the G2 phase.”