NMR solution structure and biological activity of E73, a DNA binding protein from Sulfolobus spindle virus Ragged Hills
Schlenker, Casey James.
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Sulfolobus solfataricus, a model organism for Archaea, lives in extreme thermal and acidic environments such as the hot springs of Yellowstone National Park, and is host to diverse archaeal viruses including Sulfolobus spindle shaped virus-1 (SSV1) and Sulfolobus spindle shaped virus-Ragged Hills (SSV-RH). SSV viruses exhibit remarkable morphology and genetic diversity, but are poorly understood as many proteins encoded by their genomes have very little sequence homology to proteins of known functions. Detailed structure-function studies have been undertaken to better understand the role played by SSV proteins in regulating viral gene expression, viral life cycle, and in mediating virus-host interactions. Herein, we report the 3D solution structure of E73, a 73-residue, homodimeric protein encoded within the SSV-RH genome and demonstrate its dsDNA binding capabilities. We find that E73 is comprised of an extended ribbon-helix-helix (RHH) structural domain, which is structurally homologous to the RHH domains of numerous proteins involved in regulation of gene transcription. The N-terminal beta strands of E73's protomers assemble into an anti-parallel beta-sheet, which, based on structural homology with other RHH proteins, form base-specific interactions with dsDNA. E73 is notably distinct from known RHH proteins however as it contains a third helix which forms a positively charged structural cleft that we postulate is involved in protein-protein interactions. These findings are discussed in the context of E73's potential role in regulating SSV-RH genome transcription and replication in its Sulfolobus host.