Bacteriophage GIL01 is a temperate tectivirus that infects Bacillus thuringiensis serovar israelensis. When bacteriophage GIL01 infects a cell, it establishes the lysogenic cycle and remains dormant, until the SOS response is inducted due to DNA damage. To establish lysogeny, phage GIL01 uses the bacterial transcription regulator LexA, which in complex with phage protein gp7 represses the phage lytic cycle. When the SOS response is activated, LexA undergoes autocatalytic self cleavage, which induces the lytic cycle of bacteriophage GIL01. Here we show, by using beta-galactosidase assay and surface plasmon resonance spectrometry, that LexA is not the only transcription factor regulating promotor P1 and P2 activity. We show that phage protein gp1 represses gene expression of promoter P2 and is involved in maintaining lysogeny of bacteriophage GIL01. This is the first account of bacteriophage GIL01 having its own repressor partially independent of the host SOS response. With the use of surface plasmon resonance we showed that at promoter P2, protein gp1 forms a high affinity complex. According to our in vitro data we predict that approximately 5 or 6 monomers of gp1 bind to promoter P2 via a filament that lowers promotor activity by sterically restricting access of RNA polymerase for the P2 promoter elements. With the use of beta-galactosidase assay we reconfirmed that gp7 is critical for GIL01 lysogeny. We also show that promoters P1 and P2 influence each other’s activity.
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