Department of Geosciences, Princeton University, USA.
Metallomics. 2019 Jan 23;11(1):201-212. doi: 10.1039/c8mt00236c.
To increase iron (Fe) bioavailability in surface soils, microbes secrete siderophores, chelators with widely varying Fe affinities. Strains of the soil bacterium Azotobacter chroococcum (AC), plant-growth promoting rhizobacteria used as agricultural inoculants, require high Fe concentrations for aerobic respiration and nitrogen fixation. Recently, A. chroococcum str. NCIMB 8003 was shown to synthesize three siderophore classes: (1) vibrioferrin, a low-affinity α-hydroxy carboxylate (pFe = 18.4), (2) amphibactins, high-affinity tris-hydroxamates, and (3) crochelin A, a high-affinity siderophore with mixed Fe-chelating groups (pFe = 23.9). The relevance and specific functions of these siderophores in AC strains remain unclear. We analyzed the genome and siderophores of a second AC strain, A. chroococcum str. B3, and found that it also produces vibrioferrin and amphibactins, but not crochelin A. Genome comparisons indicate that vibrioferrin production is a vertically inherited, conserved strategy for Fe uptake in A. chroococcum and other species of Azotobacter. Amphibactin and crochelin biosynthesis reflects a more complex evolutionary history, shaped by vertical gene transfer, gene gain and loss through recombination at a genomic hotspot. We found conserved patterns of low vs. high-affinity siderophore production across strains: the low-affinity vibrioferrin was produced by mildly Fe limited cultures. As cells became more severely Fe starved, vibrioferrin production decreased in favor of high-affinity amphibactins (str. B3, NCIMB 8003) and crochelin A (str. NCIMB 8003). Our results show the evolution of low and high-affinity siderophore families and conserved patterns for their production in response to Fe bioavailability in a common soil diazotroph.
为了提高表层土壤中的铁（Fe）生物利用度，微生物会分泌铁载体，这是一类具有广泛不同铁亲和力的螯合剂。土壤细菌固氮菌（AC）的菌株，作为农业接种剂使用的植物生长促进根际细菌，需要高浓度的 Fe 进行有氧呼吸和固氮。最近，A. chroococcum str. NCIMB 8003 被证明可以合成三类铁载体：（1） 弧菌血素，一种低亲和力的α-羟基羧酸（pFe = 18.4），（2） 两栖菌素，高亲和力的三羟肟酸，和（3） 钩霉素 A，一种具有混合 Fe 螯合基团的高亲和力铁载体（pFe = 23.9）。这些铁载体在 AC 菌株中的相关性和特定功能仍不清楚。我们分析了第二个 AC 菌株 A. chroococcum str. B3 的基因组和铁载体，并发现它也产生弧菌血素和两栖菌素，但不产生钩霉素 A。基因组比较表明，弧菌血素的产生是一种垂直遗传的、保守的铁吸收策略，在 A. chroococcum 和其他固氮菌属的物种中都存在。两栖菌素和钩霉素生物合成反映了一个更为复杂的进化历史，受到垂直基因转移、通过基因组热点重组发生的基因获得和丢失的影响。我们发现，在不同菌株中，低亲和力和高亲和力铁载体的产生存在保守模式：低亲和力的弧菌血素是由轻度铁限制的培养物产生的。随着细胞受到更严重的铁饥饿，弧菌血素的产生减少，有利于高亲和力的两栖菌素（B3 株、NCIMB 8003）和钩霉素 A（NCIMB 8003 株）的产生。我们的研究结果表明，在一种常见的土壤固氮菌中，低亲和力和高亲和力铁载体家族的进化以及对 Fe 生物利用度的反应存在保守模式。
