T lys , a Newly Identified Sulfolobus Spindle-Shaped Virus 1 Transcript Expressed in the Lysogenic State, Encodes a DNA-Binding Protein Interacting at the Promoters of the Early Genes

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Abstract
While studying the gene expression of the Sulfolobus spindle-shaped virus 1 (SSV1) in Sulfolobus solfataricus lysogenic cells, a novel viral transcript (T lys ) was identified. Transcriptional analysis revealed that T lys is expressed only in the absence of UV irradiation and is downregulated during the growth of the lysogenic host. The correponding gene f55 lies between two transcriptional units (T 6 and T ind ) that are upregulated upon UV irradiation. The open reading frame f55 encodes a 6.3-kDa protein which shows sequence identity with negative regulators that fold into the ribbon-helix-helix DNA-binding motif. DNA-binding assays demonstrated that the recombinant F55, purified from Escherichia coli , is indeed a putative transcription factor able to recognize site specifically target sequences in the promoters of the early induced T 5 , T 6 , and T ind transcripts, as well as of its own promoter. Binding sites of F55 are included within a tandem-repeated sequence overlapping the transcription start sites and/or the B recognition element of the pertinent genes. The strongest binding was observed with the promoters of T 5 and T 6 , and an apparent cooperativity in binding was observed with the T ind promoter. Taking together the transcriptional analysis data and the biochemical evidences, we surmise that the protein F55 is involved in the regulation of the lysogenic state of SSV1.

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