Bacteriophage E specifically recognizes and infects strains of Escherichia coli which display the alpha-2,8-linked polysialic acid K1 capsule. Bacteriophage E endosialidase, which is thought to be responsible for initial absorption of the phage to the host bacterium, was purified, and the N-terminal amino acid sequences of the polypeptide monomer and cyanogen bromide fragments were determined. Synthetic oligonucleotide probes were designed from the N-terminal amino acid sequences and used to identify restriction fragments of bacteriophage E DNA encoding the endosialidase. The primary nucleotide sequence of the bacteriophage E endosialidase gene contains an open reading frame encoding a 90 kDa polypeptide which is processed to give a mature 74 kDa protein. The native enzyme is probably a trimer of identical 74 kDa subunits. In the bacteriophage E genome the K1E endosialidase open reading frame is preceded by a putative upstream promoter region with homology to a bacteriophage SP6 promoter. A central region of 500 amino acids of the deduced protein sequence of the K1E endosialidase was found to have 84% identity to K1F endosialidase. Both endosialidases contain two copies of a sialidase sequence motif common to many bacterial and viral sialidases. These sequences flank the region of greatest identity between the two endosialidase forms, which suggests that this central domain is involved in binding and hydrolysis of the polysialic acid substrate.

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