Chatfield Christa H, Cianciotto Nicholas P
Department of Microbiology-Immunology, Northwestern University Medical School, 320 East Superior Street, Chicago, IL 60611-3010, USA.
Infect Immun. 2007 Aug;75(8):4062-70. doi: 10.1128/IAI.00489-07. Epub 2007 Jun 4.
The virulence of Legionella pneumophila is dependent upon its capacity to acquire iron. To identify genes involved in expression of its siderophore, we screened a mutagenized population of L. pneumophila for strains that were no longer able to rescue the growth of a ferrous transport mutant. However, an unusual mutant was obtained that displayed a strong inhibitory effect on the feoB mutant. Due to an insertion in hmgA that encodes homogentisate 1,2-dioxygenase, the mutant secreted increased levels of pyomelanin, the L. pneumophila pigment that is derived from secreted homogentisic acid (HGA). Thus, we hypothesized that L. pneumophila-secreted HGA-melanin has intrinsic ferric reductase activity, converting Fe(3+) to Fe(2+), but that hyperpigmentation results in excessive reduction of iron that can, in the case of the feoB mutant, be inhibitory to growth. In support of this hypothesis, we demonstrated, for the first time, that wild-type L. pneumophila secretes ferric reductase activity. Moreover, whereas the hyperpigmented mutant had increased secreted activity, an lly mutant specifically impaired for pigment production lacked the activity. Compatible with the nature of HGA-melanins, the secreted ferric reductase activity was positively influenced by the amount of tyrosine in the growth medium, resistant to protease, acid precipitable, and heterogeneous in size. Together, these data represent the first demonstration of pyomelanin-mediated ferric reduction by a pathogenic bacterium.
嗜肺军团菌的毒力取决于其获取铁的能力。为了鉴定参与其铁载体表达的基因，我们筛选了嗜肺军团菌的诱变群体，寻找那些不再能够挽救亚铁转运突变体生长的菌株。然而，我们获得了一个不寻常的突变体，它对feoB突变体表现出强烈的抑制作用。由于编码尿黑酸1,2-双加氧酶的hmgA基因插入，该突变体分泌的脓性黑素水平增加，脓性黑素是嗜肺军团菌的色素，由分泌的尿黑酸（HGA）衍生而来。因此，我们推测嗜肺军团菌分泌的HGA-黑素具有内在的铁还原酶活性，可将Fe(3+)转化为Fe(2+)，但色素沉着过度会导致铁过度还原，在feoB突变体的情况下，这可能会抑制生长。为支持这一假设，我们首次证明野生型嗜肺军团菌分泌铁还原酶活性。此外，色素沉着过度的突变体分泌活性增加，而一个专门损害色素产生的lly突变体则缺乏这种活性。与HGA-黑素的性质相符，分泌的铁还原酶活性受到生长培养基中酪氨酸量的正向影响，对蛋白酶有抗性，可被酸沉淀，且大小不均一。总之，这些数据首次证明了病原菌通过脓性黑素介导的铁还原作用。
