Praszkier Judy, Pittard A James
Department of Microbiology and Immunology, The University of Melbourne, Vic. 3010, Australia.
Plasmid. 2005 Mar;53(2):97-112. doi: 10.1016/j.plasmid.2004.12.005.
The closely related plasmids that make up the I-complex group and the more distantly related IncL/M plasmids regulate the frequency of initiation of their replication by controlling the efficiency of translation of the rate limiting replication initiator protein, RepA. Translation initiation of repA is dependent on the formation of a pseudoknot immediately upstream of its Shine-Dalgarno sequence. Formation of this pseudoknot involves base pairing between two complementary sequences in the repA mRNA and requires that the secondary structure sequestering the distal sequence be disrupted by movement of the ribosome translating and terminating a leader peptide, whose coding sequence precedes and overlaps that of repA. Expression of repA is controlled by a small antisense RNA, RNAI, which on binding to its complementary target in the repA mRNA not only pre-empts formation of the pseudoknot, but also inhibits translation of the leader peptide. The requirement that translation of the leader peptide be completed for the pseudoknot to form increases the time available for the inhibitory interaction of RNAI with its target, so that at high copy number the frequency of pseudoknot formation is lowered, reducing the proportion of repA mRNA that are translated. At low copy number, when concentration of RNAI is low, repA is translated with increased frequency, leading to increased frequency of plasmid replication.
构成I复合群的密切相关质粒以及亲缘关系较远的IncL/M质粒，通过控制限速复制起始蛋白RepA的翻译效率来调节其复制起始频率。repA的翻译起始依赖于其SD序列上游紧邻处假结的形成。该假结的形成涉及repA mRNA中两个互补序列之间的碱基配对，并且要求通过翻译并终止前导肽的核糖体移动来破坏隔离远端序列的二级结构，前导肽的编码序列位于repA之前且与之重叠。repA的表达受一种小反义RNA即RNAI的控制，RNAI与repA mRNA中的互补靶标结合时，不仅会阻止假结的形成，还会抑制前导肽的翻译。前导肽的翻译完成后假结才能形成，这增加了RNAI与其靶标发生抑制性相互作用的可用时间，因此在高拷贝数时，假结形成的频率降低，翻译的repA mRNA比例减少。在低拷贝数时，由于RNAI浓度较低，repA的翻译频率增加，导致质粒复制频率升高。
