Persistent mirusvirus infection in the marine protist Aurantiochytrium

Mirusviruses are abundant and broadly distributed double-stranded (ds) DNA viruses recently discovered in marine metagenomic data. Their host range and ecological impact are unclear. The protist Aurantiochytrium limacinum possesses two mirusvirus-like genomic elements, one a circular episome (AurliV-1) and the other (AurliV-2) a chromosomal integrant. Here we show that genes in both genomes are expressed and viral particles containing mainly AurliV-1 DNA are produced under starvation conditions and when cells are cultured in standard growth medium. We detected viral particles of ~140 nm in the nucleus, in cytoplasmic vesicles, between the plasma membrane and cell wall, and in the extracellular environment. Of 67 AurliV-1-encoded proteins detected using proteomics, 45 are enriched under starvation conditions, including the structurally important major capsid and triplex proteins. Our results establish Aurantiochytrium as a model system for elucidating mirusvirus-host interactions and demonstrate persistent viral infection in a microbial eukaryote. Mirusviruses were detected in metagenomic datasets, but little is known about how they infect their hosts. Here, the authors characterize mirusviruses in the marine protist Aurantiochytrium, detecting virions, viral genes and proteins, and establishing this as a valuable model system.