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. 2019 Mar 25;374(1772):20180087. doi: 10.1098/rstb.2018.0087

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© 2019 The Authors.

Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

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Figure 3. — Origin and evolution of class 1 CRISPR-Cas systems. The figure depicts a hypothetical scenario of the origin of class1 CRISPR-Cas from an ancestral signalling system and its subsequent evolution yielding the extant type III and type I systems, as well as reductive evolution that produced type IV systems and minimalist variants of type I system recruited by Tn7 transposons. The key evolutionary events are described to the right of the images. ‘GGDD’, a key catalytic motif of the cyclase/polymerase domain of Cas10; ‘A’, catalytically active RRM domain of a RAMP protein; RE, LE, right and left end, respectively; TR, terminal repeats. (Online version in colour.)