Partition of unit-copy miniplasmids to daughter cells. II. The partition region of miniplasmid P1 encodes an essential protein and a centromere-like site at which it acts.

The stable maintenance of the unit-copy lambda-P1:5R miniplasmid is dependent on adjacent but separable replication (rep) and partition (par) regions of DNA derived from its P1 plasmid parent. The par region consists of an approximately 2.5 X 10(3) base-pair (kb) segment of DNA of which the terminal kb contains the plasmid incompatibility determinant incB. Two of the 14 lambda-P1:5R partition-defective point mutants isolated are amber (nonsense) mutants, showing that a plasmid-encoded protein is essential for proper partition. All of the Par- point mutants are complemented by the wild-type par region in trans. The complementing activity was shown to be an Mr 44,000 protein encoded by the end of the par region distal to incB. Deletion analysis showed that the incB sequence is essential in cis to the plasmid in order that the plasmid be receptive to the par protein. Thus incB appears to be the target site for par protein activity. We propose that the protein binds to incB, forming a complex that is recognized as a substrate for the cellular partition apparatus. The ability of a cloned incB sequence to compete for the par protein or for the cellular partition apparatus accounts for its activity as an incompatibility determinant. The existence of a plasmid-encoded par protein suggests a specific model for equipartition.