CHARACTERIZATION OF MU-PROPHAGE LACKING THE CENTRAL STRONG GYRASE BINDING-SITE - LOCALIZATION OF THE BLOCK IN REPLICATION

Bacteriophage Mu contains an unusually strong DNA gyrase binding site (SGS), located near the center of its genome, that is required for eff icient Mu DNA replication (M. L. Pato, Proc, Natl. Acad. Sci. USA 91:7 056-7060, 1994; M. L. Pato, M, M, Howe, and N. P. Higgins, Proc. Natl. Acad. Sci. USA 87:8716-8720, 1990). Replication of wild-type Mu initi ates about 10 min after induction of a lysogen, while replication in t he absence of the SGS is delayed about an hour. To determine which ste p in the replication pathway is blocked in the absence of the SGS, we inactivated the SGS by deletion and by insertion and studied the effec ts of these alterations on various stages of Mu DNA replication. Follo wing induction in the absence of a functional SGS, early transcription and synthesis of the Mu-encoded replication proteins occurred normall y. However, neither strand transfer nor cleavage at the Mu genome term ini could be detected 40 min after induction. The data are most consis tent with a requirement for the SGS in the efficient synapsis of the M u prophage termini to form a separate chromosomal domain.