Copyright © 1993, American Society for Microbiology Superinfection Exclusion (sieB) Genes of Bacteriophages

The superinfection exclusion gene (sieB) of Salnonella phage P22 was mapped with phage deletion mutants. The DNA sequence in the region was reexamined in order to find an open reading frame consistent with the deletion mapping. Several discrepancies with the previously published sequence were discovered. The revised sequence revealed a single open reading frame of 242 codons with six likely translation initiation codons. On the basis of deletion and amber mutant phenotypes, the second of these six sites was inferred to be the translation initiation site of the sieB gene. The sieB gene encodes a polypeptide with 192 amino acid residues with a calculated molecular weight of 22,442, which is in reasonable agreement with that estimated from polyacrylamide gels. The transcription start site ofsieB was identified by the use of an RNase protection assay. The sieB promoter thus identified was inactivated by a 2-base substitution in its-10 hexamer. The sieB gene of coliphage A was also identified. The promoter for A sieB was identified by homology to that of P22 sieB. Lambdoid prophages encode systems that prevent the lytic growth of superinfecting phages. Thus, X and P22 are unable to grow vegetatively in X or P22 lysogens, respec-tively, because transcription from their major promoters (pL and PR) is sensitive to repression by cI (in the case of X) o