Defective Lambda Particles Whose DNA Carries Only a Single Cohesive End

Circular DNA molecules have been found in a wide variety of organisms. Although the DNA of bacteriophage λ is a linear molecule in the virus particle, it is rapidly converted into a covalently closed circular form upon injection into a bacterial cell (Young and Sinsheimer, 1964; Bode and Kaiser, 1965). Since the linear DNA molecules bear two cohesive ends (Hershey et al., 1963), this conversion is thought to result from end-joining: The ends cohere specifically to one another to form hydrogen-bonded circles which in turn are converted to covalently closed circles, probably by Escherichia coli DNA ligase. This sequence can take place in vitro (Gellert, 1967; Olivera and Lehman, 1967; Gefter et al., 1967). Endonucleolytic scission at the end-join, thought to be promoted by phage-specific functions, would regenerate the λ DNA molecule (Chapter 5). The existence of cohesive ends and the rapid in vivo formation of circles imply that circularity is a significant feature of λ DNA metabolism. Circularity has been implicated in several important phage processes—prophage integration and excision (Campbell, 1962), DNA replication (Tomizawa and Ogawa, 1968; Schnos and Inman, 1970), and late gene transcription (Herskowitz and Signer, 1970). This report concerns the identification and origin of a class of λ DNA molecules bearing only a single cohesive end, thereby providing a direct way to study the properties of λ DNA molecules unable to circularize. The gal transducing particles in lysates of induced recA ( λc 1857 int 6 red 3) differ from the typical λgal (Gottesman and Yarmolinsky, 1968b; Gingery and Echols, 1968;...