Adaptive immunity of type VI CRISPR-Cas systems associated with reverse transcriptase-Cas1 fusion proteins.

Cas13-containing type VI CRISPR-Cas systems specifically target RNA; however, the mechanism of spacer acquisition remains unclear. We have previously reported the association of reverse transcriptase-Cas1 (RT-Cas1) fusion proteins with certain types of VI-A systems. Here, we show that RT-Cas1 fusion proteins are also recruited by type VI-B systems in bacteria from gut microbiomes, constituting a VI-B1 variant system that includes a CorA-encoding locus in addition to the CRISPR array and the RT-Cas1/Cas2 adaptation module. We found that type VI RT-CRISPR systems were functional for spacer acquisition, CRISPR array processing and interference activity, demonstrating that adaptive immunity mediated by these systems can function independently of other in trans systems. We provide evidence that the RT associated with these systems enables spacer acquisition from RNA molecules. We also found that CorA encoded by type VI-B1 RT-associated systems can transport divalent metal ions and downregulate Cas13b-mediated RNA interference. These findings highlight the importance of RTs in RNA-targeting CRISPR-Cas systems, potentially enabling the integration of RNA-derived spacers into CRISPR arrays as a mechanism against RNA-based invaders in specific environments.