The archaeal lysogeny regulator F55: from discovery to in vitro and in vivo characterizations

The archaeon Sulfolobus solfataricus hosts a variety of viruses, among which the UV-inducible Sulfolobus spindle shaped virus 1 (SSV1) is the best characterized1. While studying the gene expression of SSV1 in the absence of UV irradiation2, a novel viral transcript (Tlys) was identified and found to encode for a transcription regulator, named F55, that folds into the ribbon-helix-helix DNA-binding motif. DNA band-shift assays demonstrated that F55 specifically recognises and binds to target sequences located in the promoters of the early induced SSV1 transcripts (i.e., T5, T6, and Tind) as well as in its own promoter (Tlys). The strongest affinity was observed towards T5 and T6 promoters and an apparent cooperativity in binding was observed for the Tind promoter2. Altogether, these in vitro evidences strongly indicated that F55 might be involved in the regulation of the SSV1 lysogeny and UV-induction. To shed light on the regulative role of F55, an in vivo survey of the molecular events occurring at the UV-inducible region of the SSV1 genome was carried out3. Chromatin immunoprecipitation (ChIP) assays followed by semi-quantitative PCR (sqPCR) analyses showed that F55 stably binds in vivo to the promoters of Tlys, T5, and T6 as well as to that of the UV-inducible Tind. Moreover, ChIP-sqPCR data indicated that F55 dissociates upon UV irradiation first from the promoter of Tind (2 hours post UV-irradiation) and subsequently from those of T5 and T6 (4 hours post UV-irradiation); thus, allowing their transcription to proceed3. The protein/DNA dissociation occurs because, upon UV-irradiation, the intracellular concentration of F55 becomes progressively suboptimal to saturate all the regulative binding sites, as shown by western blot analyses3. Further studies are underway to identify the molecular partners of F55 involved in the transition from the lysogenic to the UV-induced state of SSV1.