Structural and functional studies of Stf76 from the Sulfolobus islandicus plasmid-virus pSSVx: a novel peculiar member of the winged helix–turn–helix transcription factor family

The hybrid plasmid virus pSSVx from Sulfolobus islandicus presents an open reading frame encoding a 76 aminoacid protein, namely Stf76, that does not show significant sequence homology with any protein with known three-dimensional structure. The recombinant protein recognises specifically two DNA binding sites located in its own promoter, thus suggesting an auto-regulated role of its expression. CD, spectrofluorimetric, light scattering and ITC experiments indicated a 2:1 molar ratio (protein:DNA) upon binding to the DNA target containing a single site. Furthermore, the solution structure of Stf76, determined by nuclear magnetic resonance (NMR) using chemical shift Rosetta software, has shown that the protein assumes a winged helix–turn–helix fold. NMR chemical shift perturbation analysis has been performed for the identification of the residues responsible for DNA interaction. In addition, a model of the Stf76-DNA complex has been built using as template a structurally related homolog.