New insight into the biology of R-loops

R-loops form when RNA hybridizes with its template DNA generating a three-stranded structure leaving a displaced single strand non-template DNA. During transcription negative supercoiling of DNA behind the advancing RNA polymerase will facilitate the formation of R-loops by the nascent RNA as the DNA is under wound to facilitate transcription. In theory R-loops are classified into pathological and non-pathological depending on the context of its formation. R-loop which are formed normally in various physiological events like in gene regulation and at immunoglobulin class switch regions are considered non-pathological, whereas abnormally stable R-loop which leads to genomic instability are considered pathological. Although pathological R-loop formation is a rare event but once formed completely blocks transcription, mRNA export, elevates mutagenesis, and inhibits gene expression. Hence, R-loop either prevents or induces genomic instability indirectly and are potentially an endogenous source of DNA lesion. Although the existence of R-loop has been reported few decades ago, but only recently we have gained knowledge about its formation and resolution in cells due to the availability of reagents. R-loop biology has generated immense interest in past few years since it connects the important biological processes such as transcription, mRNA splicing, DNA replication, recombination and repair. In this review I will focus on the recent progress made about formation and resolution of R-loop, based on the methodologies that are currently available to study R-loop using biochemical, cell biology and molecular biology approaches.