Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) systems provide the adaptive antiviral immunity against invasive genetic elements in archaea and bacteria. These immune systems are divided into at least six different types, among which Type III CRISPR–Cas systems show several distinct antiviral activities as demonstrated from the investigation of bacterial III-A and archaeal III-B systems in the past decade. First, although initial experiments suggested that III-A systems provided DNA interference activity, whereas III-B system was active only in RNA interference, these immune systems were subsequently found to mediate the transcription-dependent DNA interference and the dual DNA/RNA interference. Second, their ribonucleoprotein (RNP) complexes show target RNA (tgRNA) cleavage by a ruler mechanism and RNA-activated indiscriminate single-stranded DNA cleavage, the latter of which is subjected to spatiotemporal regulation such that the DNase activity occurs only at the right place in the right time. Third, RNPs of Type III systems catalyse the synthesis of cyclic oligoadenylates (cOAs) that function as second messengers to activate Csm6 and Csx1, both of which are potent Cas accessory RNases after activation. To date, Type III CRISPR systems are the only known antiviral immunity that utilizes multiple interference mechanisms for antiviral defence.
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Cover Image
Cover Image
In the cover image, which depicts archaeal cell division, fl uorescence microscopy shows FtsZ1-GFP localisation in pleomorphic cells of Haloferax volcanii. This archaeal model organism was originally isolated from the Dead Sea, but is well suited to live-cell studies owing to its relatively large, flat cells that show distinct differentiation. Multiple proteins from the tubulin superfamily control cell division and cell shape in this species (see article by Liao et al in this issue; pages 547–559). Archaea are expected to provide important insights into cellular evolution and fundamental cell structures such as the cytoskeleton. Image kindly provided by Iain Duggin (The ithree institute, University of Technology Sydney).
Molecular mechanisms of III-B CRISPR–Cas systems in archaea
Nicholas P. Robinson, Yan Zhang, Jinzhong Lin, Mingxia Feng, Qunxin She; Molecular mechanisms of III-B CRISPR–Cas systems in archaea. Emerg Top Life Sci 14 December 2018; 2 (4): 483–491. doi: https://doi.org/10.1042/ETLS20180023
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