Sequencing the genomes of LPP-1, the first isolated cyanophage, and its relative LPP-2 reveal different integration mechanisms in closely related phages.

In the early 1960s, the first cyanophage was isolated. The description of this phage, named LPP-1, led to the extensive investigation of various cyanophages and to the study of their interactions with their cyanobacterial hosts towards controlling blooms. Here, the genomes of LPP-1 and its putative relative, LPP-2 were sequenced. Sequencing these genomes revealed that LPP-1 and LPP-2 are members of a group of short-tailed cyanophages, which are distantly related to the T7-like cyanophages. Most of the phages in this group have the ability to lysogenize their hosts. Their ability to switch between lytic and lysogenic infection may explain the formation of cyanobacterial blooms despite the persistence of their phages. This lysogenic capacity of the LPP-1-like phages occurs despite the lack of an obvious integrase gene within their genomes. Interestingly, we show that LPP-2 integrates into the host genome through an integration site in high proximity to a recombination endonuclease that may have integrase activity. Further understanding of cyanobacterial-phage relationships may provide insight into their population dynamics and suggest novel approaches for control of destructive cyanobacterial blooms.

Isolation and characterization of a novel Lambda-like phage infecting the bloom-forming cyanobacteria Cylindrospermopsis raciborskii.

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.