Bhatt Garima, Denny Timothy P
Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA.
J Bacteriol. 2004 Dec;186(23):7896-904. doi: 10.1128/JB.186.23.7896-7904.2004.
PhcA is a transcriptional regulator that activates expression of multiple virulence genes in the plant pathogen Ralstonia solanacearum. Relative to their wild-type parents, phcA mutants overproduced iron-scavenging activity detected with chrome azurol S siderophore detection medium. Transposon mutagenesis of strain AW1-PC (phcA1) generated strain GB6, which was siderophore negative but retained weak iron-scavenging activity. The ssd gene inactivated in GB6 encodes a protein similar to group IV amino acid decarboxylases, and its transcription was repressed by iron(III) and PhcA. ssd is the terminal gene in a putative operon that also appears to encode three siderophore synthetase subunits, a integral membrane exporter, and three genes with no obvious role in siderophore production. A homologous operon was found in the genomes of Ralstonia metallidurans and Staphylococcus aureus, both of which produce the polycarboxylate siderophore staphyloferrin B. Comparison of the siderophores present in culture supernatants of R. solanacearum, R. metallidurans, and Bacillus megaterium using chemical tests, a siderophore utilization bioassay, thin-layer chromatography, and mass spectroscopy indicated that R. solanacearum produces staphyloferrin B rather than schizokinen as was reported previously. Inactivation of ssd in a wild-type AW1 background resulted in a mutant almost incapable of scavenging iron but normally virulent on tomato plants. AW1 did not produce siderophore activity when cultured in tomato xylem sap, suggesting that the main location in tomato for R. solanacearum during pathogenesis is iron replete.
PhcA是一种转录调节因子，可激活植物病原菌青枯雷尔氏菌中多个毒力基因的表达。相对于其野生型亲本，phcA突变体在铬天青S铁载体检测培养基上检测到的铁清除活性过高。对菌株AW1-PC（phcA1）进行转座子诱变产生了菌株GB6，该菌株无铁载体活性，但保留了较弱的铁清除活性。GB6中失活的ssd基因编码一种类似于IV族氨基酸脱羧酶的蛋白质，其转录受铁（III）和PhcA抑制。ssd是一个假定操纵子中的末端基因，该操纵子似乎还编码三个铁载体合成酶亚基、一个整合膜转运蛋白以及三个在铁载体产生中无明显作用的基因。在金属抗性雷尔氏菌和金黄色葡萄球菌的基因组中发现了一个同源操纵子，这两种菌都能产生聚羧酸铁载体嗜铁素B。使用化学测试、铁载体利用生物测定、薄层色谱和质谱对青枯雷尔氏菌、金属抗性雷尔氏菌和巨大芽孢杆菌培养上清液中存在的铁载体进行比较，结果表明青枯雷尔氏菌产生的是嗜铁素B，而不是先前报道的裂褶菌素。在野生型AW1背景中使ssd失活导致一个突变体几乎无法清除铁，但在番茄植株上仍具有正常毒力。在番茄木质部汁液中培养时，AW1不产生铁载体活性，这表明青枯雷尔氏菌在发病过程中在番茄中的主要位置铁含量充足。
