Beschreibung:
<jats:p>Covalently closed extrachromosomal deoxyribonucleic acid (DNA) was isolated from alpha-hemolytic wild-type strains of<jats:italic>Escherichia coli</jats:italic>. Most strains examined were able to transfer the hemolytic property with varying frequencies to nonhemolytic recipient strains. Out of eight naturally isolated alphahemolytic<jats:italic>E. coli</jats:italic>strains, four contained a set of three different supercoiled DNAs with sedimentation coefficients of 76<jats:italic>S</jats:italic>(plasmid A), 63<jats:italic>S</jats:italic>(plasmid B), and 55<jats:italic>S</jats:italic>(plasmid C). The sedimentation coefficients and the contour lengths of the isolated molecules correspond to molecular weights of 65 × 10<jats:sup>6</jats:sup>, 41 × 10<jats:sup>6</jats:sup>, and 32 × 10<jats:sup>6</jats:sup>. Three alpha-hemolytic wild-type strains carried only one plasmid with a molecular weight of 41 × 10<jats:sup>6</jats:sup>, and one strain harbored two plasmids with molecular weights of 41 × 10<jats:sup>6</jats:sup>and 32 × 10<jats:sup>6</jats:sup>. Alpha-hemolytic transconjugants were obtained by conjugation of<jats:italic>E. coli</jats:italic>K-12 with the hemolytic wild-type strains. A detailed examination revealed that plasmids with the same sizes as plasmids B and C of the wild-type strains can be transferred separately or together to the recipients. Both plasmids possess the hemolytic determinant and transfer properties. Plasmid A appears to be, at least in one wild-type strain, an additional transfer factor without a hemolytic determinant. In one case a hemolytic factor was isolated, after conjugation, that is larger in size than plasmid A and appears to be a recombinant of both plasmids B and C.</jats:p>