中文摘要：

变性链球菌（Streptococcus mutans）是最主要的龋齿致病菌．其中Smu_195c为一个约10ku（86个氨基酸）的蛋白质，序列比对结果显示，它没有包含保守的结构域，功能也是未知的．为揭示其功能，在大肠杆菌BL21（DE3）中表达了N端结合了6个His的编码Smu_195c的DNA片段．通过悬滴法得到了两种不同晶系的晶体．第一种属于空间群P6122或P6522，晶格参数为a=b=62．93A，c=90．63A，λ=120°；第二种属于空间群P41212或P43212，晶格参数为a=b=57．97A，c=103．51A．N端带有His的蛋白质长出的晶体属于第一种，而切掉N端的6个His后的蛋白质长出的晶体两种都有．

英文摘要：

Smu_195c is a protein with 86 amino acids in Streptococcus mutans, a primary pathogen for human dental caries. The specific function of Smu_195c is still unknown and there are no conserved domains in it. In order to find out its function, the gene encodes Smu_195c was cloned and expressed in E. coli as N-terminally 6＊His tagged recombinant protein. Two crystal forms were obtained by the hanging drop method. Form Ⅰ belongs to space group P6122 or P6522 with the unit cell parameters a = b = 62.93A , c = 90.63 A, γ=120° and form Ⅱ belongs to the space group P41212 or P43212 with the unit cell parameters a =b=57.97 A, c = 103.51 A. Crystals from the protein with His-tag belong to form Ⅰ, however, crystals from the protein without His-tag belong to both.