Transposition without duplication of infecting bacteriophage Mu DNA

Abstract

Most models of DNA transposition invoke replication of the transposable element¹, but it is not clear whether a ‘co-integrate’ is an obligatory intermediate in the pathway leading to the production of simple insertions during transposition. Such an intermediate can be accounted for only by a replicative transposition scheme. Bacteriophage Mu is a temperate phage that can either lysogenize or lyse its host², and it encodes at least two modes of transposition as judged by the end-products generated by the process. During the lytic development of the integrated prophage, co-integrates are the predominant end-products³; transposition is coupled to replication during this phase⁴. A small number of simple insertions are also produced during the lytic growth³, but during transposition from the infecting phage into the host chromosome, simple insertions are the main end-products⁵. Conditions can be found where the choice between the two kinds of end-products depends on a delicate balance between the essential transposition functions encoded by Mu⁶. Experiments^7,8 have suggested that the simple insertions which arise during transposition from the infecting phage may do so without Mu DNA replication. Here I demonstrate using an infecting phage with completely methylated DNA, a dam⁻ (DNA adenine methylase) host and a combination of restriction enzymes that can cut either fully methylated or unmethylated DNA but not hemi-methylated DNA, that transposition of the phage DNA into the host chromosome does not involve a duplication of its DNA. This result may also have significance for other transposons⁹ that do not appear to go through a co-integrate intermediate during transposition¹⁰.