Genomic and proteomic characterization of two novel siphovirus infecting the sedentary facultative epibiont cyanobacterium A caryochloris marina

A caryochloris marina is a symbiotic species of cyanobacteria that is capable of utilizing far‐red light. We report the characterization of the phages A ‐ HIS 1 and A ‐ HIS 2, capable of infecting A caryochloris . Morphological characterization of these phages places them in the family S iphoviridae . However, molecular characterization reveals that they do not show genetic similarity with any known siphoviruses. While the phages do show synteny between each other, the nucleotide identity between the phages is low at 45–67%, suggesting they diverged from each other some time ago. The greatest number of genes shared with another phage (a myovirus infecting marine S ynechococcus ) was four. Unlike most other cyanophages and in common with the S iphoviridae infecting S ynechococcus , no photosynthesis‐related genes were found in the genome. CRISPR (clustered regularly interspaced short palindromic repeats) spacers from the host A caryochloris had partial matches to sequences found within the phages, which is the first time CRISPR s have been reported in a cyanobacterial/cyanophage system. The phages also encode a homologue of the proteobacterial RN ase T . The potential function of RN ase T in the mark‐up or digestion of cr RNA hints at a novel mechanism for evading the host CRISPR system.