Origin of DNA Concatemers during Growth

Abstract

Several years ago, M. G. Smith and I described a fast-sedimenting form of λ DNA, extracted from bacteria at late times after infection, which behaved like a linear molecule 2–8 times the length of DNA in λ phage particles. Results of our tracer experiments suggested that these molecules were a replicating form and the direct precursor of DNA in newly formed phage particles. Salzman and Weissbach (1967 Salzman and Weissbach (1968) confirmed our findings and showed that the conversion of fast-sedimenting DNA to finished molecules requires the function of the phage head genes A and D. Long DNA structures have been demonstrated both by electron microscopy (Weissbach et al., 1968; Tomizawa and Ogawa. 1968b; Carter et al., 1969) and by radio-autography (Carter et al., 1969). I shall use the word concatemer (Thomas et al., 1968) to refer to this form of fast-sedimenting DNA.

During early times after infection λ DNA replicates as a circle to form progeny circles (Ogawa et al., 1968; Tomizawa and Ogawa, 1968b; Young and Sinsheimer, 1967, 1968; MacKinlay and Kaiser, 1969; Kiger and Sinsheimer, 1969a; Carter et al., 1969, Schnöss and Inman, 1970). Therefore, as suggested by several of these authors, if concatemer formation is an intrinsic feature of replication, λ may possess two modes of replication: an early mode, through which progeny circles are formed, and a later mode, which generates concatemers (Kiger and Sinsheimer, 1969b). An alternative possibility is that there is only one mode of replication and concatemers are formed from progeny circles by recombination.