Novick R P, Projan S J, Kumar C C, Carleton S, Gruss A, Highlander S K, Kornblum J
Basic Life Sci. 1985;30:299-320. doi: 10.1007/978-1-4613-2447-8_24.
PT181 is a fully sequenced Staphylococcus aureus plasmid whose size is 4,437 bp. It specifies tetracycline resistance and has a copy number of about 22 per cell in exponentially growing cultures. The functional organization of the pT181 replicon is centered around the coding sequence for a 35-kd protein, RepC, that is absolutely required for replication of the plasmid. The replication origin is contained within the repC coding sequence and the region immediately 5' to the RepC start is involved in control of the plasmid replication rate. PT181 replication is controlled at the level of RepC synthesis by a negative regulatory system that is functionally similar to that of the Co1E1 and IncFII plasmids of Escherichia coli. The pT181 control circuit involves 2 short transcripts, RNA I and RNA II, that are transcribed from the region specifying the 5' end of the untranslated repC mRNA leader and in the opposite direction. These are referred to as countertranscripts. The countertranscripts regulate RepC synthesis by a mechanism that probably involves interaction with the repC mRNA leader in a manner that interferes with translation. Both of the countertranscripts seem to be necessary for normal replication control; their separate roles remain unclear. Unlike plasmids of the Co1E1 and IncFII groups, plasmids such as Co1E1 are considered to have direct regulation of replication because the inhibitory element of the copy control circuit directly inhibits the initiation of replication. Plasmids such as pT181 are considered to have indirect regulation of replication because the product of the regulated step, RepC, is trans-active. Plasmids of the IncFII type are considered to have direct regulation of replication because the product of the regulated step, RepA is cis-active The analysis of pT181 replication physiology has illustrated 2 important differences between directly and indirectly regulated plasmids: a) for directly regulated plasmids, copy mutants specifying a normal inhibitor substance but an inactive target site exclude the wild-type or recessive mutants by directly interfering with their replication. Analogous mutants of indirectly regulated plasmids coexist readily with the wild-type and all mutants (although they do manifest segregational incompatibility) because the Rep protein is always shared by all plasmids in the cell, regardless of its source. b) Mutations of directly regulated plasmids in the region where target transcript and countertranscript overlap may give rise to totally new incompatibility groups because they engender independently self-correcting copy pools.(ABSTRACT TRUNCATED AT 400 WORDS)
PT181是一个已完成全序列测定的金黄色葡萄球菌质粒，大小为4437碱基对。它赋予四环素抗性，在指数生长培养物中每个细胞的拷贝数约为22个。pT181复制子的功能组织以一种35千道尔顿蛋白质RepC的编码序列为中心，该蛋白质是质粒复制绝对必需的。复制起点包含在repC编码序列内，RepC起始位点5'端紧邻的区域参与质粒复制速率的控制。PT181复制在RepC合成水平上由一个负调控系统控制，该系统在功能上与大肠杆菌的ColE1和IncFII质粒的调控系统相似。pT181控制回路涉及2个短转录本，RNA I和RNA II，它们从指定未翻译的repC mRNA前导序列5'端的区域转录而来，且转录方向相反。这些被称为反义转录本。反义转录本通过一种可能涉及与repC mRNA前导序列相互作用从而干扰翻译的机制来调节RepC的合成。这两个反义转录本似乎都是正常复制控制所必需的；它们各自的作用仍不清楚。与ColE1和IncFII组的质粒不同，诸如ColE1这样的质粒被认为具有复制的直接调控，因为拷贝控制回路的抑制元件直接抑制复制起始。诸如pT181这样的质粒被认为具有复制的间接调控，因为被调控步骤的产物RepC具有反式活性。IncFII类型的质粒被认为具有复制的直接调控，因为被调控步骤的产物RepA具有顺式活性。对pT181复制生理学的分析揭示了直接调控和间接调控质粒之间的2个重要差异：a）对于直接调控的质粒，指定正常抑制物质但靶位点无活性的拷贝突变体通过直接干扰野生型或隐性突变体的复制来排斥它们。间接调控质粒的类似突变体很容易与野生型和所有突变体共存（尽管它们确实表现出分离不相容性），因为Rep蛋白总是由细胞中的所有质粒共享，无论其来源如何。b）直接调控质粒在靶转录本和反义转录本重叠区域的突变可能产生全新的不相容组，因为它们产生独立的自我校正拷贝池。（摘要截短至400字）
