The CRISPR/Cas9 genome-editing technique enables researchers to snip and edit bacteria DNA at will. Now, new findings reveal how bacteria take snapshots not only of DNA, but also of RNA — the first step in their process of defense against invaders.
CRISPR/Cas9 makes use of a bacterial defense system in which bacteria copy snippets of DNA into a genomic holding pen of clustered regularly interspaced short palindromic repeats (CRISPR). The bacterial machinery then transcribes those foreign DNA segments into CRISPR RNA, which acts as a guide to direct a protein called Cas9 to the identical segment of the invader’s DNA. The Cas9 protein snips the invasive DNA, disabling the intruder.
But CRISPR/Cas9 is just one CRISPR mechanism; there are at least five types of CRISPR systems, some of which coexist in a single organism. And not all Cas proteins are used for destruction; Cas1 and Cas2 proteins, in particular, are used to identify invading genomes.
In a subset of Type III CRISPR systems, the Cas1 protein is fused to an enzyme called a reverse transcriptase. The job of reverse transcriptases is to use an RNA template to generate DNA — the opposite of the typical DNA-to-RNA transcription process. The existence of this tantalizing Cas1/reverse-transcriptase duo made an...
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