Predominant end-products of prophage Mu DNA transposition during the lytic cycle are replicon fusions

Abstract

We present biochemical and genetic experiments which strongly suggest that the net result of transposition of prophage Mu and internally deleted Mu derivatives (mini-Mu's) during the lytic cycle is replicon fusion. When a Mu prophage located on pSC101, a low copy number plasmid, is induced, virtually all of the pSC101::Mu plasmid copies enter the host chromosome within 33 minutes. By cleaving the total host DNA with restriction endonucleases and by hybridization with 32P-labeled pSC101 DNA, we have found that the fused structures contain directly repeated copies of Mu at each junction of the plasmid and host DNA. These fused structures, called cointegrates, have been seen with Mu as low level genetic events and with almost all other transposable elements. Genetic analysis of mini-Mu transposition from pSC101 to an F′ pro + lac episome has also shown that transposition invariably is associated with fusion of the whole plasmid with the episome; no simple linear insertions of a mini-Mu molecule into the F′ episome were recovered. Our results also indicate that jumping of Mu from a chromosomal location onto an F′ episome rarely results in the linear insertion of a single copy of Mu DNA; instead, the episome apparently first undergoes Mu-mediated fusion with the chromosome followed by release of Mu-containing episomes that carry extensive deletions or insertions. Integration of Mu into an F′ episome during lysogenization, however, leads to simple point insertions of the Mu genome. Our data strongly suggest that the end-products of Mu DNA transposition during prophage induction and during lysogenization are not the same. We infer, therefore, that the process of Mu DNA transposition can occur by either of two alternate pathways which differ with respect to the end-products they generate.