P1 plasmid replication: multiple functions of RepA protein at the origin.

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Abstract
Replication functions of a bacteriophage P1 miniplasmid are carried on a 1.2-kilobase pair (kb) segment that can be subdivided into a 245-base pair (bp) replication origin and a 959-bp region that encodes a protein required for replication (RepA). The origin region contains five 19-bp direct repeats. By using primer extension and gene-fusion assays, the promoter of the repA gene was mapped within the repeated sequences and showed that the promoter is repressed by RepA. Regulation of RepA synthesis apparently achieved by the binding of RepA to the repeat sequences. This regulation might be a key step in the replication-control circuit, as it was found that overproduction of RepA (from a foreign promoter) inhibits replication. Thus, in addition to being an autoregulated activator of replication, the protein also can have a negative regulatory role.