Switch in the transposition products of Mu DNA mediated by proteins: Cointegrates versus simple insertions

Bacteriophage Mu is a self-contained mobile unit encoding functions that mediate its movement. There appear to be two alternate pathways for Mu DNA transposition that differ with respect to the end products they generate. During the lytic cycle of phage Mu growth the end products of transposition are predominantly cointegrates in an experimental system in which the induced Mu prophage is located on pSC101, a low-copy-number plasmid. On the other hand, Mu insertions into the host genome during lysogenization contain Mu DNA as simple insertions. Two Mu functions, encoded by the A and B genes, are required for Mu DNA transposition during its lytic growth. However, during lysogeny the product of gene B is not required for integration of Mu DNA. Evidence is presented here which shows that in the absence of the B gene product the majority of transposition events are simple insertions. This is in striking contrast to the situation in which the majority of the products are cointegrates in the presence of both A and B gene products. Additional evidence also suggests that these simple insertions do not arise through the resolution of cointegrate structures.