Hernday Aaron D, Braaten Bruce A, Low David A
Biomolecular Sciences and Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106, USA.
Mol Cell. 2003 Oct;12(4):947-57. doi: 10.1016/s1097-2765(03)00383-6.
The expression of pyelonephritis-associated pili (Pap) in uropathogenic Escherichia coli is epigenetically controlled by a reversible OFF to ON switch. In phase OFF cells, the global regulator Lrp is bound to pap sites proximal to the pilin promoter, whereas in phase ON cells, Lrp is bound to promoter distal sites. We have found that the local regulator PapI increases the affinity of Lrp for the sequence "ACGATC," which contains the target "GATC" site for DNA adenine methylase (Dam) and is present in both promoter proximal and distal sites. Mutational analyses show that methylation of the promoter proximal GATC(prox) site by Dam is required for transition to the phase ON state by specifically blocking PapI-dependent binding of Lrp to promoter proximal sites. Furthermore, our data support the hypothesis that PapI-dependent binding of Lrp to a hemimethylated GATC(dist) site generated by DNA replication is a critical component of the switch mechanism.
致病性大肠杆菌中肾盂肾炎相关菌毛（Pap）的表达受表观遗传调控，存在一个从关闭到开启的可逆开关。在关闭阶段的细胞中，全局调节因子Lrp与菌毛蛋白启动子近端的pap位点结合，而在开启阶段的细胞中，Lrp与启动子远端位点结合。我们发现局部调节因子PapI增加了Lrp对序列“ACGATC”的亲和力，该序列包含DNA腺嘌呤甲基化酶（Dam）的靶标“GATC”位点，且存在于启动子近端和远端位点。突变分析表明，Dam对启动子近端GATC（prox）位点的甲基化是通过特异性阻断Lrp与启动子近端位点的PapI依赖性结合而转变为开启状态所必需的。此外，我们的数据支持这样一种假说，即Lrp与DNA复制产生的半甲基化GATC（dist）位点的PapI依赖性结合是开关机制的关键组成部分。
