Bacteriophage GIL01 is a temperate phage of the Tectiviridae family that infects isolates of Bacillus thuringiensis. Th0e GIL01 phage genome is a linear DNA that replicates like an autonomous replicon during a lysogenic cycle. When the SOS response is activated in bacteria after genome damage, the phage becomes lytic. Two phage proteins are required to maintain the lysogenic cycle – gp1 and gp7, as well as the bacterial repressor LexA. Protein gp1 characteristics are not well known. We isolated the protein gp1 without or in a complex with nonspecific DNA, which forms a complex with the protein as an impurity during protein isolation. By analyzing the interactions between the molecules with surface plasmon resonance, we confirmed that protein gp1 binds to the P2 promoter region of bacteriophage GIL01. The nucleotide motif of two direct repeats downstream of the -10 element of promoter P2 is important for gp1 binding. Based on our results, we suggest that the direct repeat sequence of the operator functions as a nucleation site. First, the protein gp1 binds to this specific site and then it polymerizes upstream to cover the P2 promoter and represes it. On average, seven molecules of protein gp1 interact with promoter P2 and most likely form a filament on DNA. Using cross-linking and protein resolution on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, we conclude that protein gp1 does not form oligomers when free in solution. We build a three-dimensional model of the structure of protein gp1 and suggest that protein gp1 consists of three ɑ-helixes, three ꞵ-sheets, and five loops.
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