Capsid Structure of a Freshwater Cyanophage Siphoviridae Mic1

Cyanobacteria are the most abundant photosynthetic microorganisms, the global distribution of which is mainly regulated by the corresponding cyanophages. A systematic screening of water samples in the Lake Chaohu enabled us to isolate a freshwater siphocyanophage that infects Microcystis wesenbergii, thus termed Mic1. Using cryoelectron microscopy, we solved the 3.5-Å structure of Mic1 capsid. The major capsid protein gp40 of an HK97-like fold forms two types of capsomers, hexons and pentons. The capsomers interact with each other via the interweaved N-terminal arms of gp40 in addition to a tail-in-mouth joint along the three-fold symmetric axis, resulting in the assembly of capsid in a mortise-and-tenon pattern. The novel-fold cement protein gp47 sticks at the two-fold symmetric axis and further fixes the capsid. These findings provide structural insights into the assembly of cyanophages, and set up a platform to explore the mechanism of specific interactions and co-evolution with cyanobacteria. [Display omitted] •A new freshwater long-tailed cyanophage Mic1 was identified from Lake Chaohu•The capsid structure of Mic1 at 3.5-Å resolution was solved by cryo-EM•Mic1 capsid adopts a mortise-and-tenon assembly mechanism The cryo-EM structure of a freshwater cyanophage Mic1 capsid reveals the structural variations of the major capsid protein and a novel fold of the cement protein, which enable the assembly of the icosahedral capsid via a mortise-and-tenon pattern.