Temperature-modulated persistent dsRNA phage infection selectively excludes superinfection and promotes generation of...

Persistent viral infections are common in nature and can play a significant role in the ecology and evolution of the host organism. However, our understanding of these prolonged virus-host interactions remains limited. This study characterizes the persistent infection strategy of Pseudomonas phage phi6, an enveloped virus with a tri-segmented, double-stranded (ds)RNA genome that infects Pseudomonas syringae. Phi6 is predominantly lytic but can alternatively establish a non-productive chronic infection (carrier cell state) in which viral particles assemble and replicate viral RNA without causing host cell lysis and viral progeny release. We utilized reverse genetics tools to produce stable phi6 carrier cells and identified genetic factors essential for carrier cell formation. We found that the persistent phi6 infection does not significantly impact host growth at the optimal growth temperature. Nevertheless, growth temperature may regulate the switch between the carrier cell state and lytic infection. In addition, the persistent phi6 infection protects the host bacterium against secondary phi6 infections, providing an apparent fitness benefit for the host. Gene knockout assays suggested that phi6 gene 8, encoding the viral major outer capsid protein P8, contributes to the superinfection exclusion effect. However, the persistently infected bacterial cells remained susceptible to other dsRNA phages, enabling genetic exchange between related phages. Accordingly, P8 of phi6 specifically inhibited transcription by phi6 nucleocapsids but did not suppress transcription of a related phage in an in vitro transcription assay. This study sheds light on the underlying molecular mechanisms and potential ecological implications of persistent dsRNA phage infection in plant pathogenic Pseudomonas.

Double-stranded (ds)RNA viruses infect a wide range of hosts, including bacteria, fungi, plants, and animals, and this group includes many important pathogens. While acute infections of these viruses are well understood, less is known about dsRNA virus persistence. We studied persistent infection of a bacterial dsRNA virus, cystovirus phi6, in Pseudomonas syringae. In this virus-host interaction, intracellular phage particles are continuously produced without host cell lysis, deviating from other known persistent phage infection strategies. Our findings show that persistent dsRNA virus infection can benefit the host: persistence does not affect host growth but protects it from subsequent phi6 infections. Nevertheless, the persistently infected host remains susceptible to other viruses, promoting the formation of new viral genotypes, and highlighting the importance of persistent infections in viral evolution. Furthermore, understanding the carrier cell phenomenon is relevant for the development of phi6-based biocontrol of Pseudomonas infections in crops.