Characterization of emb, a Gene Encoding the Major Adhesin of Streptococcus defectivus

Streptococcus defectivus is one of the nutritionally variant streptococci, a class of viridans group streptococci first isolated from patients with endocarditis and otitis media. In previous studies, NVS-47, a clinical isolate of S. defectivus, was shown to bind to the extracellular matrix. A high-molecular-weight surface protein was identified and proposed to be responsible for mediating this binding. In the present study, the gene encoding this protein was identified by transposon mutagenesis and characterized. The gene (emb) was found to be larger than 14 kb and was partially sequenced. It encodes a protein containing at least 50 repeats of 77 amino acids predicted to assume an alternating coiled-coil conformation. The domain responsible for extracellular matrix binding was mapped to the N terminus of the protein. From sequence analysis, Emb is proposed to be the prototype of a new family of streptococcal fibrillar proteins.

缺陷链球菌（Streptococcus defectivus）是营养变异链球菌（nutritionally variant streptococci）的成员之一，该类群属于草绿色链球菌群（viridans group streptococci），最初从心内膜炎与中耳炎患者体内分离得到。既往研究显示，缺陷链球菌的临床分离株NVS-47可结合细胞外基质（extracellular matrix）。研究人员鉴定出一种高分子量表面蛋白，并推测其介导了该结合过程。本研究通过转座子诱变（transposon mutagenesis）技术，对编码该蛋白的基因进行了鉴定与表征。该基因（emb）长度大于14 kb，已完成部分测序，其编码的蛋白包含至少50个77个氨基酸的重复序列，经预测该蛋白呈现交替卷曲螺旋（coiled-coil）构象。细胞外基质结合结构域被定位至该蛋白的N端。通过序列分析，研究人员推测Emb蛋白是链球菌纤维状蛋白新家族的原型成员。