Abstract
Holliday junctions (HJs) are DNA intermediates in homology-directed DNA repair and replication stalling, but until recently were undetectable in living cells. We review how an engineered protein that traps and labels HJs in Escherichia coli illuminates the biology of DNA and cancer. HJ chromatin immunoprecipitation with deep sequencing (ChIP-seq) analysis showed the directionality of double-strand break (DSB) repair in the E. coli genome. Quantification of HJs as fluorescent foci in live cells revealed that the commonest spontaneous problem repaired via HJs is replication-dependent single-stranded DNA gaps, not DSBs. Focus quantification also indicates that RecQ DNA helicase plays dual roles in promoting repair HJs and preventing replication-stall HJs in an E. coli model of RAD51-overexpressing (most) cancers. Moreover, cancer transcriptomes imply that most cancers suffer frequent fork stalls that are reduced by the HJ removers EME1 and GEN1, as well as by the human RecQ orthologs BLM and RECQL4-surprising potential procancer roles for these known cancer-preventing proteins.