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We cloned the DNA containing the Escherichia coli hemolysin determinant on a small, high-copy plasmid. We generated plasmids containing fragments of this DNA and used them either alone or in two-plasmid complementation systems to define the limits of the structural genes. This system also allowed us to partially characterize the function of each of the gene products in the production and transport of hemolysin. Taken with previously published data, the present experiments indicate the following. (i) At least three cistrons, hlyC, hlyA, and hlyB (these were previously designated cisC, etc. [Noegel et al., Mol. Gen. Genet. 175:343-350, 1979]), contain the specific genetic information for the hemolytic phenotype, (ii) hlyA encodes a 107,000-kilodalton protein, which seems to be an inactive precursor of hemolysin. (iii) Normal amounts of hemolysin activity inactive precursor of hemolysin. (iii) Normal amounts of hemolysin activity require only the products of hlyA and hlyC. This activity was found in the periplasm; very little hemolysin activity was found in the cytoplasm, suggesting that the hlyC product is required for transport or activation of the hlyA product or both. (iv) Active hemolysin remains in the periplasm in the absence of hlyB function, hence the hlyB product seems to be necessary for the transport of hemolysin to the exterior of the cell. We further show that overproduction of the hlyA product is lethal, probably causing lysis of the cell.

Cloning and functional characterization of the plasmid-encoded hemolysin determinant of Escherichia coli