Hybridization of RNA to double-stranded DNA: formation of R-loops.

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Abstract
RNA [from yeast] can hybridize to double-stranded [.lambda.gt-Scllog] DNA in the presence of 70% formamide by displacing the identical DNA strand. The resulting structure, called an R-loop, is formed in formamide probably because of the greater thermodynamic stability of the RNA.cntdot.DNA hybrid when it is near the denaturation temperature of duplex DNA. The rate of R-loop formation is maximal at the temperature at which half of the duplex DNA is irreversibly converted to single-stranded DNA (the strand separation temperature or tss) of the duplex DNA and falls precipitously a few degrees above or below that temperature. This maximal rate is similar to the rate of hybridization of RNA to single-stranded DNA under the same conditions. At temperatures above the tss the rate is proportional to the RNA concentration. At temperatures below tss the rate of R-loop formation is less dependent upon the RNA concentration. Once formed, the R-loops display considerable stability; the formamide can be removed and the DNA can be cleaved with restriction endonucleases without loss of R-loop structures.