Abstract

 

Prokaryotes encode clustered regularly interspaced short palindromic repeat (CRISPR) arrays and CRISPR-associated (Cas) genes as an adaptive immune machinery. CRISPR-Cas systems effectively protect hosts from the invasion of foreign enemies, such as bacteriophages and plasmids. Non-self nucleic acid fragments are acquired as spacers between repeats in the host CRISPR array during a process called í«adaptationí» to establish immunological memory. The highly conserved Cas1-Cas2 complexes function as molecular recorders to integrate spacers in a time course manner, which can subsequently be expressed as crRNAs complexed with Cas effector proteins for the RNA-guided interference pathways. In some of RNA-targeting type III systems, Cas1 proteins are fused with reverse transcriptase (RT), indicating that RT- Cas1-Cas2 complexes can acquire RNA transcripts for spacer acquisition. In this review, we summarize current studies focused on the molecular structure and function of the RT-fused Cas1-Cas2 integrase and its potential applications as a directional RNA-recording tool in cells. Furthermore, we highlight outstanding questions for RT-Cas1-Cas2 studies and future directions for RNA-recording CRISPR technologies.