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. 2019 Sep 13;10:2160. doi: 10.3389/fmicb.2019.02160

FIGURE 4.

FIGURE 4

Origin of the type VI-A/RT1 and type VI-A/RT2 subtypes. The figure depicts a hypothetical scenario for the origin of adaptation modules for type VI-A systems containing an associated RT. Different adaptation modules containing RT-Cas1 and Cas2 proteins were captured independently, probably from type III-D systems, by phylogenetically different Cas13a-containing type VI CRISPR-Cas systems. Note that the interference module of type III systems encodes a multi-subunit effector complex and that the analysis of metagenomic datasets revealed that a variant of the type VI-A/RT1 system containing an Cas6-RT-Cas1 fusion may exist in uncultivated microorganisms.