Jiao Yongqin, Newman Dianne K
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
J Bacteriol. 2007 Mar;189(5):1765-73. doi: 10.1128/JB.00776-06. Epub 2006 Dec 22.
Phototrophic Fe(II)-oxidizing bacteria couple the oxidation of ferrous iron [Fe(II)] to reductive CO(2) fixation by using light energy, but until recently, little has been understood about the molecular basis for this process. Here we report the discovery, with Rhodopseudomonas palustris TIE-1 as a model organism, of a three-gene operon, designated the pio operon (for phototrophic iron oxidation), that is necessary for phototrophic Fe(II) oxidation. The first gene in the operon, pioA, encodes a c-type cytochrome that is upregulated under Fe(II)-grown conditions. PioA contains a signal sequence and shares homology with MtrA, a decaheme c-type cytochrome from Shewanella oneidensis MR-1. The second gene, pioB, encodes a putative outer membrane beta-barrel protein. PioB is a homologue of MtrB from S. oneidensis MR-1. The third gene, pioC, encodes a putative high potential iron sulfur protein (HiPIP) with a twin-arginine translocation (Tat) signal sequence and is similar to the putative Fe(II) oxidoreductase (Iro) from Acidithiobacillus ferrooxidans. Like PioA, PioB and PioC appear to be secreted proteins. Deletion of the pio operon results in loss of Fe(II) oxidation activity and growth on Fe(II). Complementation studies confirm that the phenotype of this mutant is due to loss of the pio genes. Deletion of pioA alone results in loss of almost all Fe(II) oxidation activity; however, deletion of either pioB or pioC alone results in only partial loss of Fe(II) oxidation activity. Together, these results suggest that proteins encoded by the pio operon are essential and specific for phototrophic Fe(II) oxidation in R. palustris TIE-1.
光合铁（II）氧化细菌利用光能将亚铁离子[Fe（II）]的氧化与还原性二氧化碳固定耦合起来，但直到最近，人们对这一过程的分子基础仍知之甚少。在这里，我们以沼泽红假单胞菌TIE-1作为模式生物，报告了一个三基因操纵子的发现，该操纵子被命名为pio操纵子（用于光合铁氧化），它是光合铁（II）氧化所必需的。操纵子中的第一个基因pioA编码一种c型细胞色素，在Fe（II）生长条件下会上调表达。PioA含有一个信号序列，与来自腐败希瓦氏菌MR-1的十聚体c型细胞色素MtrA具有同源性。第二个基因pioB编码一种假定的外膜β-桶蛋白。PioB是来自腐败希瓦氏菌MR-1的MtrB的同源物。第三个基因pioC编码一种具有双精氨酸转运（Tat）信号序列的假定高电位铁硫蛋白（HiPIP），并且与来自嗜酸氧化亚铁硫杆菌的假定Fe（II）氧化还原酶（Iro）相似。与PioA一样，PioB和PioC似乎也是分泌蛋白。pio操纵子的缺失导致Fe（II）氧化活性丧失以及在Fe（II）上生长的能力丧失。互补研究证实该突变体的表型是由于pio基因的缺失。单独缺失pioA几乎导致所有Fe（II）氧化活性丧失；然而，单独缺失pioB或pioC仅导致Fe（II）氧化活性部分丧失。总之，这些结果表明pio操纵子编码的蛋白质对于沼泽红假单胞菌TIE-1中的光合铁（II）氧化是必不可少且具有特异性的。
