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

Seven families of double-stranded DNA viruses have been identified, among which the Fuselloviridae and Rudiviridae are the most well-studied specimens and therefore represent model systems for detailed studies of archaeal virus biology. Two distinct genetic elements, SSV2 and pSSVx, belong to Fuselloviridae and coexist in the same Sulfolobus islandicus REY15/4 host, thus representing one of the few known two-virus systems in Archaea (1). pSSVx is a satellite virus that generates virus particles with the help of SSV2-associated packaging mechanisms. The transcriptional pattern of pSSVx undergoes a temporal variation of gene expression during its own life cycle, thus providing a good model for studying regulation of gene expression in Archaea (2). This genetic element encodes four TFs possibly implicated in the regulation of gene expression, i.e. ORF-c68, ORF51, ORF91 and ORF76. Among these, ORF76, here named Stf76 (Sulfolobus transcription factor 76 aminoacid protein), has homologs in almost all conjugative and cryptic plasmids from Sulfolobus (3), thus suggesting a relevant role for this protein in replication and/or maintenance of the plasmid. In this study, we have performed a detailed structural and functional characterization of Stf76. The corresponding gene has been cloned, expressed in Escherichia coli and the recombinant protein purified to homogeneity. To elucidate its interaction with the identified DNA operator sequence, analyses regarding its DNA binding capabilities by means of EMSA, circular dichroism, spectrofluorimetric and isothermal titration calorimetry experiments have been performed. Moreover, a structural study has been undertaken by Nuclear Magnetic Resonance spectroscopy leading to:(i) the solution structure of Stf76 based on CS-Rosetta approach, (ii) the characterization of the Stf76-DNA interaction by chemical shift perturbation analysis, (iii) a structural model describing the interaction of a single Stf76 monomer with its DNA operator. Altogether these results contribute to elucidate the regulatory mechanism underpinning the role of this protein (4). REFERENCES: 1. ARNOLD, H.P., ET AL.(1999) THE GENETIC ELEMENT PSSVX OF THE EXTREMELY THERMOPHILIC CRENARCHAEON SULFOLOBUS IS A HYBRID BETWEEN A PLASMID AND A VIRUS. MOL MICROBIOL, 34, 217-226. 2. CONTURSI, P., ET.AL. (2010) TRANSCRIPTION TERMINATION IN THE PLASMID/VIRUS HYBRID PSSVX FROM SULFOLOBUS ISLANDICUS. EXTREMOPHILES, 14, 453-463. 3. LIPPS, G. (2006) PLASMIDS AND VIRUSES OF THE THERMOACIDOPHILIC CRENARCHAEOTE SULFOLOBUS. EXTREMOPHILES, 10, 17-28. 4. CONTURSI P., FARINA B., PIRONE L. ET AL. NUCLEIC ACIDS RES. ACCEPTED 28 FEB. 2014.