Archaeal viruses display a wide range of viral morphologies and replication cycles, which are largely distinct from eukaryotic and bacterial viruses. Whereas the infections cycles of many bacterial and eukaryotic viruses have since long been characterized in detail, those of archaeal viruses remain largely unexplored terrain. Recently, studies on a few model archaeal viruses such as SIRV2 (Sulfolobus islandicus rod shaped virus 2) revealed an unusual lysis mechanism, involving the formation of large pyramidal egress structures on the host cell surface. These 200 nm structures are built of a single virus encoded protein and open at the end of the infection cycle to release the viral progeny. Prior to this event, the virus successfully replicates its linear dsDNA genome and assembles its rod shaped viral particles in the cytoplasm.
To increase understanding of the infection cycle of SIRV2 we have employed RNAseq to monitor transcription of host and viral genes at several stages of the infection process. Besides pronounced expression of the structural proteins, at the early stages of infection we found high transcription levels of a small virus encoded protein. The protein has a predicted HTH motif and we hypothesized that due to the high expression levels it might play an important role in the early stages of the infection cycle. We solved the crystal structure of this protein and biochemically characterized its DNA binding behavior with electrophoretic mobility shift assays and atomic force microscopy. Surprisingly the protein was not only binding DNA, but also extremely condensing it. As a consequence, the protein is toxic for the viral host Sulfolobus. This toxicity could be traced back to a single protein domain. With this combinatory biochemical and in vivo approach we have revealed another archaeal viral protein important for SIRV2 infection. This adds to our understanding of the astonishing archaeal infection cycles.