Unusual Guide-binding Pockets in RNA-targeting pAgo Nucleases.

Argonaute nucleases use small nucleic acid guides to recognize and degrade complementary nucleic acid targets. Most prokaryotic Argonautes (pAgos) recognize DNA targets and may play a role in cell immunity against invader genetic elements. We have recently described two related groups of pAgo nucleases that have distinct specificity for DNA guides and RNA targets (DNA > RNA pAgos). Here, we describe additional pAgos from the same clades of the pAgo tree and demonstrate that they have the same unusual nucleic acid specificity. The two groups of DNA > RNA pAgos have non-standard guide-binding pockets in the MID domain and differ in the register of guide DNA binding and target cleavage. In contrast to other pAgos, which coordinate the 5'-end of the guide molecule by their C-terminal carboxyl, DNA > RNA pAgos have an extended C-terminus located away from the MID pocket. We show that modifications of the C-terminus do not affect guide DNA binding, but inhibit cleavage of complementary and mismatched RNA targets by some DNA > RNA pAgos. Our data suggest that the unique C-terminus found in DNA > RNA pAgos can modulate their catalytic properties and can be used as a target for pAgo modifications.