Improved methods for the formation and stabilization of R-loops for visualization in the electron microscope are presented. The two complementary strands of a duplex DNA are photochemically crosslinked once every 1 to 3 kb using 4, 5′, 8 trimethylpsoralen. R-loops are then formed by incubation with RNA in 70% formamide at a temperature above the DNA melting temperature. Finally, the R-loops are stabilized by modifying the free single strand of DNA with glyoxal, thus minimizing the displacement of the hybridized RNA by branch migration. In this manner R-loops can be formed and visualized at a high frequency irrespective of the base composition of the nucleic acid of interest.
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