Dittmann E, Erhard M, Kaebernick M, Scheler C, Neilan B A, von Döhren H, Börner T
Institute for Biology (Genetics), Humboldt University, Chausseestr. 117, 10115 Berlin, Germany.
Microbiology (Reading). 2001 Nov;147(Pt 11):3113-9. doi: 10.1099/00221287-147-11-3113.
Microcystin is a potent inhibitor of eukaryotic protein phosphatases and has been implicated in causing hepatotoxicity to humans and animals worldwide. It is produced primarily by the bloom-forming cyanobacterium Microcystis aeruginosa, although the function of the peptide in this micro-organism is unknown. In this study, a microcystin-related protein, MrpA, was identified using a microcystin-lacking mutant of M. aeruginosa, PCC 7806. Comparative two-dimensional protein electrophoresis showed that MrpA was strongly expressed in wild-type PCC 7806, but was not detectable in the mcyB mutant. MrpA showed similarity to the RhiA protein from Rhizobium leguminosarum, which is encoded by the rhiABC operon and controlled by quorum-sensing mediators. Sequencing of mrpA flanking regions in M. aeruginosa PCC 7806 revealed the presence of a rhiB homologue, mrpB, directly downstream of mrpA. Northern blot analyses of mrpA expression in cells exposed to different light conditions revealed a rapid decline of transcription under high light conditions. Most striking was a strong increase in transcript levels from cultures irradiated with blue light. The mrpA transcription level was strongly reduced in two independent microcystin-lacking mutants under all light conditions investigated.
微囊藻毒素是真核生物蛋白磷酸酶的强效抑制剂，在全球范围内已被证实会对人类和动物造成肝毒性。它主要由形成水华的铜绿微囊藻产生，尽管该肽在这种微生物中的功能尚不清楚。在本研究中，利用铜绿微囊藻PCC 7806的一个缺乏微囊藻毒素的突变体鉴定出一种与微囊藻毒素相关的蛋白MrpA。二维蛋白质电泳比较显示，MrpA在野生型PCC 7806中强烈表达，但在mcyB突变体中无法检测到。MrpA与豆科根瘤菌的RhiA蛋白相似，后者由rhiABC操纵子编码并受群体感应介质控制。对铜绿微囊藻PCC 7806中mrpA侧翼区域的测序揭示了在mrpA直接下游存在一个rhiB同源物mrpB。对暴露于不同光照条件下的细胞中mrpA表达的Northern印迹分析显示，在高光条件下转录迅速下降。最显著的是，蓝光照射培养物中的转录水平大幅增加。在所有研究的光照条件下，两个独立的缺乏微囊藻毒素的突变体中mrpA转录水平均显著降低。
