Seek and Destroy: Single-molecule perspective on the target search and recognition by the Cas9 endonuclease

The most commonly known relationship between the macromolecules of life – DNA, RNA and protein – is the central dogma which states that information flows from DNA to RNA through transcription and from RNA to protein through translation. The discovery of non-coding RNAs has altered and expanded this picture by revealing many more functions of RNA as well as different ways nucleic acids and proteins interact. For example, tRNAs act as a link between mRNA and ribosomes in translation, ribosomal RNA together with proteins make up the main translation machinery, riboswitches enable mRNA to regulate its own activity, miRNA and siRNA regulate gene expression through RNA interference. A notable recent addition to this list is the adaptive prokaryotic CRSIPR immune system where RNA molecules guide proteins to destroy the invading viral DNA. The CRISPR systems were made famous by the discovery of the Cas9 endonuclease, a single RNA-guided protein, which can perform target search, recognition and cleavage without other proteins involved. This has made Cas9 the golden tool for gene editing, making the latter cheaper, faster and easier than it has ever been before.