Terminal repetition in the DNA of bacteriophage T5

Abstract

The DNA of bacteriophage T5 can be shown to be terminally repetitious by annealing after partial digestion with λ exonuclease. Under these conditions circular molecules are formed both before and after repair of the natural singlechain interruptions with polynucleotide ligase. Repaired molecules, however, require less digestion with λ exonuclease and yield a higher frequency of circles than unrepaired DNA. These differences, which are not due to digestion at the internal sites in unrepaired DNA, suggest that a single-chain interruption is located within the terminal repetition. This hypothesis was confirmed by an electron microscopic examination of circular molecules prepared after extensive digestion with λ exonuclease. These molecules contain short, internal duplex segments which represent the complementary chains which annealed during circle formation. In repaired DNA the duplex segments are of uniform length and give an estimate of 10,000 base pairs (9% of the genome) for the size of the terminal repetition. The duplex segments in unrepaired circular molecules, however, are heterogeneous in length and, in most cases, shorter than the terminal repetition. A single-chain interruption can thus be located at variable positions within the repetition at one end of T5 DNA. The repetition at the other end of the molecule does not appear to contain an interruption.