Interaction of a Putative Transcriptional Regulatory Protein and the Thermo-inducible cts-52 Mutant Repressor in the Bacillus subtilis Phage φ105 Genome

A 144 amino acid residue cts-52 mutant repressor (mtcφ105) located in the EcoRI-F immunity region (immF) of Bacillus subtilis phage φ105 is involved in the control mechanism of a thermo-inducible expression system. Adjacent to the repressor gene, an open-reading frame, designated ORF4, encodes a polypeptide of 90 amino acid residues, which shares a 37% homology with the amino acid sequence of the repressor. On the basis of the protein sequence alignment, a DNA-binding α helix–β turn–α helix (HTH) motif was identified in the N-terminal region (residues 18–37) of the repressor as well as in the polypeptide of ORF4 (residues 22–41). In vivo expression of the mutant repressor and ORF4 were confirmed by real-time reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis. To study their DNA binding properties, the wild-type repressor (wtcφ105) and the mutant repressor mtcφ105, which has a Thr17 to Ile substitution, were overexpressed in Escherichia coli and purified for affinity assays. Their affinities towards six operator sites at various temperatures were elucidated by surface plasmon resonance (SPR). Our data showed that a temperature shift does not influence the wtcφ105–operatorsʹ binding affinity, while the binding of mtcφ105 to the operators was temperature sensitive. This explains how thermo-induction triggers the release of the mutant repressor and renders heterologous gene expression. Interestingly, mtcφ105 and ORF4 demonstrated a large affinity discrepancy towards individual operators at different temperatures. mRNA levels monitored by real-time RT-PCR indicated a suppression of mtcφ105 expression, but a stimulation of ORF4 transcription after thermo-induction. Our data suggested that ORF4 might be a counter protein to the phage repressor in the modulation of the two divergent-oriented promoters PM and PR within the immF region.