LAUSR

Summary Cylindrospermopsis raciborskii is a central bloom‐forming cyanobacteria. However, despite its ecological significance, little is known of its interactions with the phages that infect it. Currently, only a single sequenced genome of a Cylindrospermopsis‐infecting phage is publicly available. Here we describe the isolation and characterization of Cr‐LKS3, a second phage infecting Cylindrospermopsis. Cr‐LKS3 is a siphovirus with a higher genome similarity to prophages within heterotrophic bacteria genomes than to any other cyanophage/cyano‐prophage, suggesting that it represents a novel cyanophage group. The function, order and orientation of the 72 genes in the Cr‐LKS3 genome are highly similar to those of Escherichia virus Lambda (hereafter Lambda), despite the very low sequence similarity between these phages, showing high evolutionary convergence despite the substantial difference in host characteristics. Similarly to Lambda, the genome of Cr‐LKS3 contains various genes that are known to be central to lysogeny, suggesting it can enter a lysogenic cycle. Cr‐LKS3 has a unique ability to infect a host with a dramatically different GC content, without carrying any tRNA genes to compensate for this difference. This ability, together with its potential lysogenic lifestyle shed light on the complex interactions between C. raciborskii and its phages.