The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-CRISPR-associated (Cas) system is an adaptive immune system in bacteria and archaea that resists exogenous invasion through nucleic acid-mediated cleavage. In the type III-A system, the Csm complex contains five effectors and a CRISPR RNA, which edits both single stranded RNA and double stranded DNA. It has recently been demonstrated that cyclic oligoadenylates (cOAs), which are synthesized by the Csm complex, act as second messengers that bind and activate Csm6. Here, we report the crystal structures of Staphylococcus epidermidis Csm3 (SeCsm3) and an N-terminally truncated Csm6 (SeCsm6ΔN) at 2.26 and 2.0 Å, respectively. The structure of SeCsm3 highly resembled previously reported Csm3 structures from other species; however, it provided novel observations allowing further enzyme characterization. The homodimeric SeCsm6ΔN folds into a compact structure. The dimerization of the HEPN domain leads to the formation of the ribonuclease active site, which is consistent with the reported Csm6 structures. Altogether, our studies provide a structural view of the ribonuclease activity mediated by Csm3 and Csm6 of the type III-A CRISPR-Cas system.

Clustered Regularly Interspaced Short Palindromic R epeats ( CRISPR ) - C RISPR-相关( Cas ) 系统是细菌和古细菌中的一种适应性免疫系统，可通过核酸介导的切割抵抗外源入侵。在 III-A 型系统中，Csm 复合体包含五个效应子和一个 CRISPR RNA，它可以编辑单链 RNA 和双链 DNA。最近已经证明，由 Csm 复合物合成的环状低聚腺苷酸 (cOA) 充当结合和激活 Csm6 的第二信使。在这里，我们报告了表皮葡萄球菌的晶体结构Csm3 (SeCsm3) 和 N 末端截短的 Csm6 (SeCsm6ΔN) 分别位于 2.26 和 2.0 Å。SeCsm3 的结构与之前报道的其他物种的 Csm3 结构高度相似；然而，它提供了新的观察结果，可以进一步表征酶。同源二聚体 SeCsm6ΔN 折叠成一个紧凑的结构。HEPN 结构域的二聚化导致核糖核酸酶活性位点的形成，这与报道的 Csm6 结构一致。总之，我们的研究提供了由 III-A 型 CRISPR-Cas 系统的 Csm3 和 Csm6 介导的核糖核酸酶活性的结构视图。