The sim gene of Escherichia coli phage P1: nucleotide sequence and purification of the processed protein

The sim gene of bacteriophage P1 causes exclusion of a superinfecting P1 phage. We determined the nucleotide sequence of a 1.9-kb DNA fragment that, in plasmids, causes Sim phenotype. There are two open reading frames within this region for proteins of 82 and 259 amino acids. A 1.3-kb fragment containing the larger open reading frame was inserted into an expression vector. Induced cells carrying the hybrid plasmid, termed pBD5, were not infected by phage P1 and produced a 24-kDa protein and, to a smaller extent, a 25-kDa protein. The 24-kDa protein was purified. Comparison of its amino-terminal amino acid sequence with the nucleotide sequence indicated that it is processed from a precursor protein by removal of a hydrophobic leader peptide of 20 amino acids. In vivo processing depends on secA gene function and is necessary for Sim interference with P1 infection. The data are discussed with respect to the function of the sim gene in superinfection exclusion.