Koepsel R R, Khan S A
Nucleic Acids Res. 1987 May 26;15(10):4085-97. doi: 10.1093/nar/15.10.4085.
RepC protein encoded by plasmid pT181 has single-stranded endonuclease and topoisomerase-like activities. These activities may be involved in the initiation (and termination) of pT181 replication by a rolling circle mechanism. RepC protein cleaves the bottom strand of DNA within the origin of replication at a single, specific site when the DNA is in the supercoiled or linear (double or single-stranded) form. We have found that RepC protein will also cleave single-stranded DNA at sites other than the origin of replication. We have mapped the secondary cleavage sites on pT181 DNA. When the DNA is in the supercoiled, or linear, double-stranded form, only the primary site within the origin is cleaved. However, when the DNA is present in the single-stranded form, several strong and weak cleavage sites are observed. The DNA sequence at these cleavage sites shows a strong similarity with the primary cleavage site. The presence of Escherichia coli SSB protein inhibited cleavage at all of the secondary nick sites while the primary nick site remained susceptible to cleavage.
质粒pT181编码的RepC蛋白具有单链内切核酸酶和拓扑异构酶样活性。这些活性可能通过滚环机制参与pT181复制的起始（和终止）。当DNA处于超螺旋或线性（双链或单链）形式时，RepC蛋白在复制起点内的单个特定位点切割DNA的底部链。我们发现RepC蛋白也会在复制起点以外的位点切割单链DNA。我们已经绘制了pT181 DNA上的二级切割位点。当DNA处于超螺旋或线性双链形式时，仅切割起点内的主要位点。然而，当DNA以单链形式存在时，会观察到几个强切割位点和弱切割位点。这些切割位点的DNA序列与主要切割位点具有很强的相似性。大肠杆菌SSB蛋白的存在抑制了所有二级切口位点的切割，而主要切口位点仍然易于切割。
