Ends of bacteriophage mu DNA.

Abstract

THE temperate bacteriophage Mu does not prefer a specific attachment site on the chromosome of Escherichia coli and can apparently engineer the insertion of its DNA irrespective of the host sequences encountered1. The highly promiscuous, non-homologous integrative recombination between Mu DNA and E. coli DNA has prompted a comprehensive analysis of the structure of the Mu genome. The DNA molecules in mature Mu virus particles are linear duplexes of 37–38 kilobases (kb) (ref. 2). When the two Mu DNA strands are separated and reannealed, complete duplexes are not regenerated. Instead, one end is always split into two single-stranded tails, which can be readily seen with an electron microscope3,4. This end of Mu DNA has been identified as the right end or the S end5. The single-stranded tails are generally 1.5 kb in length, although occasionally the single strand can be as short as 0.5 kb or as long as 3.0 kb (ref. 6). The non-renaturation of the S end clearly reflects the heterogeneity of the terminal sequences. Studies on the renaturation kinetics of the Mu DNA have implied that E. coli sequences at the S end are responsible for the heterogeneity7, Allet and Bukhari have proposed that the left end of Mu DNA, the c end, is also not fixed and varies in length by about 100 base pairs8. This proposal stemmed from the observation that the DNA fragment cleaved from the c end of Mu by the restriction endo-nuclease HindIII (from Hemophilus influenzae) does not give a sharp band on polyacrylamide or Agarose gels after electrophoresis. The heterogenity of the c end is normally not detectable by electron microscopic techniques.