Wexler M, Yeoman K H, Stevens J B, de Luca N G, Sawers G, Johnston A W
School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK.
Mol Microbiol. 2001 Aug;41(4):801-16. doi: 10.1046/j.1365-2958.2001.02556.x.
In the N2-fixing bacterium Rhizobium leguminosarum, mutations in a homologue of tonB (tonB(Rl)) block the import of vicibactin and haem as iron sources in free-living bacteria. TonB(Rl) mutants were normal for growth with ferric dicitrate and slightly reduced for growth with haemoglobin as sole iron sources. The deduced TonB(Rl) product is larger than that of (for example) Escherichia coli, on account of an extended N-terminal domain. Transcription of tonB(Rl) was enhanced in low-Fe growth conditions; this was not controlled by Fur, nor RpoI, an Fe-regulated extracytoplasmic sigma factor. Upstream of tonB(Rl) and transcribed divergently is an operon, hmuPSTUV, whose products are homologous to ABC transporters involved in haem uptake in pathogenic bacteria. Expression of hmuPSTUV was enhanced in low-Fe conditions, and hmu mutants show slightly diminished growth on haem as sole Fe source, suggesting that there is more than one system for the uptake of this molecule. hmuPSTUV expression appears to be from three closely linked promoters. Downstream of hmuPSTUV, a gene that may encode an extracytoplasmic sigma factor was identified, but this gene, rpoZ, did not affect the transcription of tonB(Rl) or hmuPSTUV. Mutations in tonB(Rl), hmu genes and rpoZ did not affect symbiotic N(2) fixation in peas.
在固氮细菌豆科根瘤菌中，tonB同源物（tonB(Rl)）发生突变会阻碍作为铁源的维西菌素和血红素在自由生活细菌中的导入。tonB(Rl)突变体在以柠檬酸铁为铁源生长时表现正常，而以血红蛋白为唯一铁源生长时略有减少。由于N端结构域的延伸，推导的TonB(Rl)产物比例如大肠杆菌的产物更大。tonB(Rl)的转录在低铁生长条件下增强；这不受Fur或RpoI（一种铁调节的胞外sigma因子）的控制。tonB(Rl)上游且转录方向相反的是一个操纵子hmuPSTUV，其产物与参与病原菌血红素摄取的ABC转运蛋白同源。hmuPSTUV的表达在低铁条件下增强，hmu突变体在以血红素为唯一铁源时生长略有减弱，这表明存在不止一种摄取该分子的系统。hmuPSTUV的表达似乎来自三个紧密相连的启动子。在hmuPSTUV下游，鉴定出一个可能编码胞外sigma因子的基因，但该基因rpoZ不影响tonB(Rl)或hmuPSTUV的转录。tonB(Rl)、hmu基因和rpoZ的突变不影响豌豆中的共生固氮。
