Neuwald A F, Stauffer G V
Department of Microbiology, University of Iowa, Iowa City 52242.
Gene. 1989 Oct 30;82(2):219-28. doi: 10.1016/0378-1119(89)90047-4.
The smp gene, whose promoter divergently overlaps with the Escherichia coli serB promoter, encodes a 24-kDa membrane protein of unknown function. An smp fusion to the lacZ gene was constructed to select for smp promoter up-mutations. The mutations isolated, however, were located within the smp structural gene rather than within the promoter region. A 15-bp deletion and two different point mutations, corresponding to the hydrophobic region of the Smp signal sequence, resulted in a Lac+ phenotype. The point mutations introduced a positively charged amino acid into the hydrophobic region. The Smp signal sequence is unique in that it is encoded by an mRNA which has the potential to form a 9-bp hairpin structure with a 39-nt loop, that conforms to the idealized mirror symmetric sequence GC(RGUGR)(YUGUY)5 (RGUGR)CG. This highly symmetrical region may encode additional intragenic information important for the expression of smp. A computer search revealed that the smp gene product shares homology with elongation factor Ts and ribosomal subunit L4, two components of the E. coli translational apparatus.
smp基因的启动子与大肠杆菌serB基因的启动子反向重叠，该基因编码一种功能未知的24 kDa膜蛋白。构建了smp与lacZ基因的融合体以筛选smp启动子的上位突变。然而，分离出的突变位于smp结构基因内而非启动子区域。一个15 bp的缺失和两个不同的点突变，对应于Smp信号序列的疏水区域，导致了Lac+表型。点突变在疏水区域引入了一个带正电荷的氨基酸。Smp信号序列的独特之处在于它由一种mRNA编码，该mRNA有可能形成一个带有39 nt环的9 bp发夹结构，符合理想化的镜像对称序列GC(RGUGR)(YUGUY)5 (RGUGR)CG。这个高度对称的区域可能编码对smp表达重要的额外基因内信息。计算机搜索显示，smp基因产物与延伸因子Ts和核糖体亚基L4具有同源性，这是大肠杆菌翻译装置的两个组成部分。
