Plasmid P1 replication: negative control by repeated DNA sequences.

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Abstract
The incompatibility locus, incA, of the unit-copy plasmid P1 is contained within a fragment that is essentially a set of 9 19-base-pair repeats. One or more copies of the fragment destabilizes the plasmid when present in trans. Extra copies of incA interfere with plasmid DNA replication and that a deletion of most of incA increases plasmid copy number. Thus, incA is not essential for replication but is required for its control. When cloned in a high-copy-number vector, pieces of the incA fragment that each contain only 3 repeats destabilize P1 plasmids efficiently. This result makes it unlikely that incA specifies a regulatory product. Evidently, the repeating DNA sequence itself negatively controls replication by titrating a P1-determined protein, RepA, that is essential for replication. Consistent with this hypothesis is the observation that the RepA protein binds to the incA fragment in vitro.