Type I 限制修饰（R-M）系统是细菌的重要分子机器。它通过对寄主DNA特异序列的识别与甲基化修饰限制外源DNA依赖水平基因转移（horizontal gene tranfer，HGT）侵入寄主染色体，从而保证遗传的稳定性。特别是在控制致病菌耐药性的产生和变迁方面起到了关键作用。.因此，测定这个分子机器的结构和组装方式是研究其作用机理的根本。但是，目前还没有关于Type I R/M系统亚基的二元、三元复合物晶体三.维结构的报道。本项目旨在通过X-ray 晶体学方法解析Type I DNA甲基转移酶 M2S1、M2S1-DNA复合物、以及限制修饰酶M2R2S1的三维结构，并结合生物化学、分子生物学开展功能研究，进一步探讨结构和功能的关系。通过这些工作，揭示Type I R/M 分子机器的装配与作用机制，为.阐明其阻碍HGT的分子机理，控制致病菌获得抗生素抗性的遗传工程与应用奠定基础。

Type I DNA restriction-modification(R-M)system are molecular machines found in over halfbacterial genomes,including many pathogens. They restrict the influx of foreign DNA via horizontal gene transfer(HGT)into the bacterium while maintaining sequence-specific methylation modification of host DNA, and maintain the integrity of bacterial genomes. Special important in many pathogens, Type I R-M system plays a role in controlling the spread of drug resistance ,such as multidrug-resistant Staphylococcus aureus(MRSA),where the Sau1 type I R-M system appears to regulate HGT. An important goal in understanding Type I R-M system -- The large molecular machines is determination of their molecular architecture,as this provides the foundation for understanding their mechanism. Type I R-M enzymes is a complex oligemer formed from a core methyltransferase(MTase)comprised one HsdS DNA Specificity subunit and two HsdM DNA Modification subunits, M2S1, with a total molecular weight of -160kDa.This is complexed with two additional HsdR DNA Restriction subunits,giving a complex R2M2S1 with a total molecular weight of 440kDa that is capable of functioning as both MTase and restriction endonulease. The overall crystal structure is not known for either the MTase or the complete R-M complex althrough several models have been proposed.The absence of these structures has hindered study of Type I R/M enzymes but their enduring appeal as research targets is due to their extraordinary complexity of operation. This project aims to resolving the three dimensional structrue of M2S1, M2S1-DNA, and Type I restriction/modification enzymes M2R2S1 complexes by X-ray crystallographic method, combined with biochemistry, molecular biology research on function, to further explore the relationship between structure and function. Through these work, reveal the molecular architecture and action mechanism of Type I R-M molecular machines, to further clarify the molecular mechanism of block HGT, and lay the foundation for the genetic engineering development in the control of pathogenic bacteria acquired antibiotic resistance.