An intermediate in the replication of bacteriophage T7 DNA molecules

Abstract

During the intracellular DNA synthesis of several species of phage, a rapidly-sedimenting, shear-fragile structure can be identified. T7 displays this characteristic type of intracellular structure. In this report we show (a) that the rapidly-sedimenting form found in T7-infected cells is, in fact, T7 DNA, (b) that it represents a true intermediate in the production of progeny T7 DNA molecules, and (c) that it sediments more rapidly because it is longer than mature T7 DNA. Molecular length distributions obtained by electron microscopic measurements agree with molecular length distributions calculated from sedimentation profiles assuming all molecules are linear duplexes. Some of the longest molecules visualized in the electron microscope are three to four times the length of mature T7 molecules. Although the proof is not complete, we conclude that these long DNA molecules are a series of repeating genomes (concatemers). Some models are proposed to account for the production of these concatemers and for their ultimate conversion to non-permuted, terminally repetitious mature T7 DNA molecules.