Skarstad K, Baker T A, Kornberg A
Department of Biochemistry, Stanford University School of Medicine, CA 94305-5307.
EMBO J. 1990 Jul;9(7):2341-8. doi: 10.1002/j.1460-2075.1990.tb07406.x.
The Escherichia coli initiator protein, dnaA complexed with ATP, binds to the replication origin (oriC) and melts the duplex in the AT-rich part of oriC. Opening of the duplex requires a minimum temperature and superhelix density. A low level of HU protein (5 molecules/oriC) is stimulatory, probably by facilitating DNA bending. High levels of HU protein (136 molecules/oriC) restrain the free superhelicity. Under adverse conditions for oriC melting (i.e. absence of HU, high levels of HU, low temperature or low superhelicity), transcription can activate initiation of replication by generating an R-loop located even a large distance from oriC. The strand-separation stage at oriC prior to the entry of the dnaB helicase was specifically stimulated. Activation by the R-loop was blocked when a stretch of 14 GC base pairs was inserted between the loop and oriC. Thus, activation of oriC by the distantly located R-loop appears to require propagation of DNA melting through the intervening sequence.
大肠杆菌起始蛋白dnaA与ATP结合后，会与复制起点（oriC）结合，并使oriC富含AT的部分双链解链。双链的打开需要最低温度和超螺旋密度。低水平的HU蛋白（5个分子/oriC）具有促进作用，可能是通过促进DNA弯曲来实现的。高水平的HU蛋白（136个分子/oriC）会限制自由超螺旋度。在oriC解链的不利条件下（即没有HU、高水平的HU、低温或低超螺旋度），转录可以通过产生一个即使距离oriC很远的R环来激活复制起始。在dnaB解旋酶进入之前，oriC处的链分离阶段会受到特异性刺激。当在环和oriC之间插入一段14个GC碱基对时，R环的激活就会被阻断。因此，远距离的R环对oriC的激活似乎需要DNA解链通过中间序列进行传播。
