Fig. 5. I-loops are induced by strand damage at telomeric repeats (see also Supplementary Fig. 6).

a 2D-gels showing that the t-circle arc can be strongly induced by formation of nicks and gaps. MEFs nuclei were incubated with 0; 1; 2.5 or 5 µg/ml of DNase I for 8 min at RT. The reaction was stopped, the DNA was separated in 2D-gels, blotted on a membrane and hybridized with a telomeric probe. The signal ratio in the t-circle arc (yellow arrows) and in the linears (black arrows), is reported relative to the untreated sample, which was arbitrarily set to 100. Source data are provided as a Source Data File. b 2D-gels showing that the t-circle arc can form spontaneously, in the presence of nicks and gaps at telomeres. Isolated mouse DNA was incubated with 0; 0.1; 0.2 or 0.4 µg/ml of DNase I for 8 min at RT. The reaction was stopped, the DNA was separated in 2D-gels, blotted on a membrane and hybridized with a telomeric probe. The ratio of the signal in the t-circle arc and in the linears is reported relative to the untreated sample, which was arbitrarily set to 100. Source data are provided as a Source Data File. c Dot blot showing the enrichment of telomeric repeats, after large-scale DNase I treatment. Around 500 × 10⁶ SV40LT-immortalized MEFs nuclei were incubated either with 0 or 5 µg/ml of DNase I for 8 min at RT. The reaction was stopped and telomeres were enriched with the procedure described in Fig. 1. The indicated amounts from each enrichment step were spotted on a membrane and hybridized with a telomeric probe. The signal per ng of DNA is reported relative to the non-enriched DNA. Source data are provided as a Source Data File. d Accumulation of i-loops at telomeres damaged by DNase I. Telomere-enriched DNA from the experiment described in (c) was analyzed in EM. The percentage of molecules containing i-loops is reported. A KpnI-digested bulk genomic DNA control was included for the sample treated with DNase I. e Examples of molecules with i-loops observed at telomere preparations from DNase I-treated nuclei. Insets show 2X enlargements of the area inside the yellow rectangles.