Ravnum S, Andersson D I
Department of Microbiology, Uppsala University, Sweden.
Mol Microbiol. 1997 Jan;23(1):35-42. doi: 10.1046/j.1365-2958.1997.1761543.x.
Expression of the Salmonella typhimurium btuB gene, which encodes an outer membrane protein required for vitamin B12 uptake, is repressed by the presence of external vitamin B12. We have, by means of a mutational analysis, investigated which btuB sequences are required for repression. Analysis of btuB::lacZ transcriptional and translational fusions of various lengths showed that the control was exerted mainly at the translational level and required both coding and leader sequences in the btuB transcript. Regulatory mutants with a B12 non-repressible phenotype were isolated and the mutations were shown to be located at several sites within the btuB leader. Analysis of constructs carrying site-directed point mutations, which either destabilized or restabilized a putative RNA hairpin that sequesters the btuB ribosomal binding site, demonstrated that this hairpin was essential for normal repression. Comparison of the S. typhimurium btuB gene with the previously characterized S. typhimurium cbiA and Escherichia coli btuB genes reveals significant similarities as well as differences in the cis-acting sequences required for repression.
鼠伤寒沙门氏菌的btuB基因编码一种摄取维生素B12所需的外膜蛋白，其表达会受到胞外维生素B12的抑制。我们通过突变分析，研究了btuB的哪些序列是抑制作用所必需的。对不同长度的btuB::lacZ转录和翻译融合体的分析表明，这种调控主要在翻译水平上发挥作用，并且需要btuB转录本中的编码序列和前导序列。分离出了具有B12不可抑制表型的调控突变体，这些突变被证明位于btuB前导区内的几个位点。对携带定点突变的构建体进行分析，这些突变要么使隔离btuB核糖体结合位点的假定RNA发夹结构不稳定，要么使其重新稳定，结果表明该发夹结构对于正常抑制作用至关重要。将鼠伤寒沙门氏菌的btuB基因与先前已表征的鼠伤寒沙门氏菌cbiA基因和大肠杆菌btuB基因进行比较，结果显示在抑制所需的顺式作用序列方面既有显著相似性也存在差异。
