原型间隔区相邻基序 (PAM)

前间隔序列邻近基序（PAM）的发现是理解CRISPR-Cas系统如何如此精准运作的关键时刻。研究人员观察到，该系统要成功靶向并切割外源DNA，目标序列旁边必须存在一段特定的短序列（即前间隔序列）。对于广泛使用的来自化脓性链球菌的Cas9蛋白而言，该序列为5'-NGG-3'，其中'N'可以是任何核苷酸。Cas9蛋白携带其向导RNA，首先扫描DNA以寻找该PAM序列。只有当Cas9蛋白与PAM序列结合后，才会尝试解开相邻的DNA，并检测其是否与向导RNA序列匹配。如果匹配成功，Cas9的核酸酶结构域将被激活，从而产生双链断裂。

This PAM-dependent targeting mechanism is the key to how the system avoids attacking the bacterium’s own genome. The CRISPR array, from which the guide RNAs are derived, contains the same spacer sequences as the targets. However, the repeat sequences within the CRISPR array do not contain the PAM sequence. Consequently, the Cas9-gRNA complex cannot bind stably to the CRISPR locus itself, preventing an autoimmune catastrophe. This elegant solution for self/non-self discrimination is a hallmark of the system’s efficiency. For gene editing applications, the PAM requirement is a double-edged sword: it ensures specificity but also constrains the set of possible target sites in a genome. This has driven significant research into discovering or engineering Cas variants with different, more flexible, or even non-existent PAM requirements to make any part of the genome accessible to editing.