Isolation and genomic characterization of two bacteriophages that infect strain IMCC1322, a representative isolate of the SAR116 clade of marine Alphaproteobacteria

초록

Marine bacteria play crucial roles in global biogeochemical cycling. Bacteriophages have been known to mediate a substantial proportion of bacterial mortality in marine habitats. Therefore, studies on specific phage-host bacteria systems of marine environments would be helpful to understand the marine microbial dynamics and its impacts on biogeochemical cycles. In this study, we have isolated and characterized two lytic phages that infect ‘Candidatus Puniceispirillum marinum’ IMCC1322, a representative cultured strain of the SAR116 clade. The SAR116 clade is an alphaproteobacterial group widely distributed in the euphotic zone of oceans. Both phages were isolated from the East Sea of Korea, where the host strain had been cultured. Phage HMO-2011 was screened using a liquid culture-based assay. Another phage, named vB_PmaP_P1322B (briefly, P1322B), was isolated by enrichment culture followed by plaque assay. Morphological examination of phage particles using transmission electron microscope showed that both phages belong to the Podoviridae, with a short tail and ≈60 nm sized isometric head. Genome sequences of the phages were determined by Sanger sequencing (HMO-2011) and Illumina sequencing (P1322B). Genomes of two phages were very similar in length (55~56 Kb) and G+C content (43~44 mol%), and showed a high degree of sequence similarity over ≈80% of genome length. Among ≈80 genes predicted from each genome, two phages shared 57 genes, including genes for helicase, DNA polymerase, and ribonucleotide reductase. DNA methyltransferase was found only in the P1322B genome, while an auxiliary metabolic gene encoding methanesulfonate monooxygenase was predicted only from the HMO-2011 genome.