Primary structure of the immI immunity region of bacteriophage P22

The DNA sequence of the immI immunity region of bacteriophage P22 has been determined. This region includes the ant gene, which encodes the P22 antirepressor protein, and the mnt and arc genes, which encode proteins that negatively regulate antirepressor synthesis. We have purified antirepressor protein and selected tryptic peptides of antirepressor, and have determined the amino terminal sequences and amino acid composition of these molecules. These data, in combination with the DNA sequence, locate the ant gene and define the complete amino acid sequence of antirepressor (300 residues). The mnt and arc genes have been located by sequencing the mnt-am343 and arc-amH1605 mutations. The Mnt and Arc proteins are predicted to be small, basic polypeptides that are homologous in amino acid sequence. The Mnt protein also shows significant sequence homology with the lambda Cro protein. The arc and ant genes are transcribed rightward from the Pant promoter, while mnt is transcribed leftward from a promotor that may overlap Pant. The Mnt protein apparently acts by binding to an operator site located immediately adjacent to the startpoint of Pant transcription.