Instability of transposase activity: evidence from bacteriophage mu DNA replication

Transposition of genetic elements involves coupled replication and integration events catalyzed in part by a class of proteins called transposases. We have asked whether the transposase activity of bacteriophage Mu (the Mu A protein) is stable and capable of catalyzing multiple rounds of coupled replication/integration, or whether its continued synthesis is required to maintain Mu DNA replication. Inhibition of protein synthesis during the lytic cycle with chloramphenicol inhibited Mu DNA synthesis with a half-life of approximately 3 min, demonstrating a need for continued protein synthesis to maintain Mu DNA replication. Synthesis of specific Mu-encoded proteins was inhibited by infecting a host carrying a temperature-sensitive suppressor, at permissive temperature, with Mu amber phages, then shifting to nonpermissive temperature. When Aam phages were used, Mu DNA replication was inhibited with kinetics essentially identical to those with chloramphenicol addition; hence, it is likely that continued synthesis of the Mu A protein is required to maintain Mu DNA replication. The data suggest that the activity of the Mu A protein is unstable, and raise the possibility that the Mu A protein and other transposases may be used stoichiometrically rather than catalytically.