Host specificity of DNA produced byEscherichia coli

Summary Phage , whose DNA was physically labelled with density markers²H and¹⁵N and biologically labelled with K-host specificity, was submitted to one growth cycle on a restriction deficient (r^–) strain ofE. coli B exerting normally its modification function (m_B⁺). All progeny phages, including those with nonreplicated, fully conserved parental DNA molecules, acquired the B-specificity in this passage, independently of DNA replication and of the presence of the K-specificity on the DNA. Phages with parental DNA had preserved the parental K-specificity. However, about half of the phages carrying a halfheavy DNA molecule (corresponding presumably to a semiconserved double helix) did only plate on B and onr^– mutants, but not on K12. Experimental evidence is presented, that DNA degradation is the cause of this lack of growth in K12, while in infections initiated by the other half of the hybrids both strands (that with K and B specificity and that with only B specificity) are preserved and are recovered in the progeny with equal chance.