R-loop

Schematic representation of factors promoting R-loop formation and stabilization

An R-loop is a three-stranded nucleic acid structure, composed of a DNA:RNA hybrid and the associated non-template single-stranded DNA. R-loops may be formed in a variety of circumstances. They may be tolerated or cleared by cellular components. The name reflects the similarity of these structures to D-loops; the "R" in this case represents the involvement of an RNA moiety.

In the laboratory, R-loops can be created by transcription of DNA sequences (for example those that have a high GC content) that favor RNA annealing behind the progressing RNA polymerase. At least 100 base pairs (bp) of DNA:RNA hybrid is required to form a stable R-loop. R-loops may be created by hybridization of mature mRNA with double-stranded DNA under conditions favoring the formation of a DNA-RNA hybrid; in this case, the intron regions (spliced out of the mRNA) form single-stranded DNA loops, as they cannot hybridize with complementary mRNA sequences.

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