A previously undescribed archaeal virus suppresses host immunity

Extremophilic archaea can have chronic viral infections that are well-tolerated by the hosts and may potentially protect against more lethal infections. Here we show that a natural Haloferax strain (48N), is chronically infected by a lemon-shaped virus. This viral infection is not cleared spontaneously, despite the multiple defense systems of the host. Curing 48N of its virus led to radical changes in the gene expression profile of 48N and a dramatic improvement in its growth rate. Remarkably, the cured 48N is the fastest-growing haloarchaeon reported to date, with a generation time of ~107 min at 45 °C, and faster than any known Haloferax species at this temperature. The virus subverts host defenses by reducing its transcription, including the CRISPR spacer acquisition machinery. Nonetheless, even in the virus-cured background, spacer acquisition is very low, indicating that another genetic element is disrupting CRISPR activity. Our results suggest that the slow growth of some halophilic archaea could be due to the effects of proviruses within their genomes that consume resources and alter the gene expression of their hosts.