Nucleotide sequence and mutational analysis of an immunity repressor gene from Bacillus subtilis temperate phage phi 105.

We have identified and sequenced a bacteriophage phi 105 gene encoding an immunity repressor, the first to be characterized from a temperate phage infecting a Gram-positive host. Using superinfection immunity as an assay for repressor function, the phi 105 repressor gene was located within a 740-bp PvuII-HindIII subfragment near the left end of the phi 105 EcoRI-F fragment. We show that the repressor is specified by the 5'-proximal coding sequence of a translationally overlapping gene pair, transcribed from right to left on the conventional phi 105 map. Comparison of its amino acid sequence (146 residues) with that of a large number of Gram-negative bacterial and phage repressors revealed a putative DNA-binding region between positions 20 and 39. The coding region is preceded by a strong Shine-Dalgarno sequence 5' AAAGGAG 3'. Deletion analysis of the 5'-flanking DNA allowed to identify transcriptional control elements. Their structure, 5' TTGTAT 3' at -35 and 5' TATAAT 3' at -10, strongly suggests that the phi 105 repressor gene is transcribed by the major vegetative form of B. subtilis RNA polymerase, as would be expected for an early phage gene.