Figure 1.

Regulation of pIP501 replication. (A) Working model on regulation of pIP501 replication. The minimal replicon with the copR and repR genes is shown, separated by the 329-bp long leader region. (B) Dual function of CopR. On the one hand, CopR represses transcription from the repR promoter pII by binding to its operator region upstream of the −35 box, thereby inhibiting RNAP binding. On the other hand, CopR prevents convergent transcription from pII and pIII that, in its absence reduces initiation at the supercoiling-sensitive antisense promoter pIII which results in lower RNAIII compared to RNAII levels, yielding a lower attenuation rate, and, hence, higher replication rate. Additionally, the higher amount of RNAII (repR mRNA) titrates the remaining long-lived RNAIII, which further reduces the amount of the inhibitor. In the presence of CopR, RNAP transcribes less frequently through pIII, which allows higher initiation rates at pIII resulting in increased premature termination of RNAII transcription and, consequently, lower replication rates. Left, the plasmid-copy number is 10-fold lower than right, but relatively more RNAIII is present, the amount of RNAIII is (as determined in Northern blots) approximately the same in both cases, reflected by the same thickness of the red arrows symbolizing RNAIII.