Rts1 RepA and P1 RepA are trans-acting proteins essential for initiation of replication of Rts1 and P1 plasmids, respectively. We recently found that P1 RepA bound in vitro to the Rts1 replication origin as strongly as Rts1 RepA and activated the origin in vivo. However, the ori activation was quite inefficient. This study shows that by replacing a small region of P1 RepA with the corresponding region of Rts1 RepA, the efficiency of Rts1 ori activation increased markedly. Interestingly, the same subregion of P1 RepA was found to be important for in vivo activation of the P1 origin. Thus, a region essential for efficient activation of the replication origin was assigned to the P1 RepA molecule as well as to the Rts1 RepA molecule. The region was distinct from a domain necessary for in vitro binding to the origin, although both regions were required for in vivo activation of the respective origin.
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