Function Programme

Highlights • We report the crystal structure of PbuCas13b at 1.65 Å resolution • We present and validate models for RNA targeting and cleavage • We rationally engineer Cas13b to change RNA cleavage specificity • Cas13b is structurally unrelated to other CRISPR systems Summary Type VI CRISPR-Cas systems contain programmable single-effector RNA-guided RNases, including Cas13b, one of the four known family members.

INTRODUCTION Cyanophages can exert significant control on cyanobacterial population dynamics, influencing species turnover, diversity, and evolution (1–3) and, thereby, the biogeochemical cycling and other functions of the aquatic ecosystems (4, 5).

While filamentous cyanobacteria play a crucial role in food web dynamics and biogeochemical cycling of many aquatic ecosystems around the globe, the knowledge regarding phages infecting them is limited. Here we describe the complete genome of the virulent cyanophage vB_AphaS-CL131 (CL 131), a Siphoviridae phage that infects filamentous diazotrophic bloom forming cyanobacterium Aphanizomenon flos-aquae in the brackish Baltic Sea.