A second [2Fe-2S] ferredoxin from Sphingomonas sp strain RW1 can function as an electron donor for the dioxin dioxygenase

The first step in the degradation of dibenzofuran and dibenzo-p-dioxin by S phingomonas sp. strain RW1 is carried out by dioxin dioxygenase (DxnA1A2), a ring-dihydroxylating enzyme. An open reading frame (fdx3) that could pote ntially specify a new ferredoxin has been identified downstream of dxmA1A2, a two-cistron gene (J. Armengaud, B. Happe, and K. N. Timmis, J. Bacteriol . 180:3954-3966, 1998). In the present study, we report a biochemical analy sis of Fdx3 produced in Escherichia coli. This third ferredoxin thus far id entified in Sphingomonas sp. strain RW1 contained a putidaredoxin-type [2Fe -2S] cluster which was characterized by UV-visible absorption spectrophotom etry and electron paramagnetic resonance spectroscopy. The midpoint redox p otential of this ferredoxin (E'(o) -247 +/- 10 mV versus normal hydrogen el ectrode at pH 8.0) is similar to that exhibited by Fdx1 (-245 mV), a homolo gous ferredoxin previously characterized in Sphingomonas sp. strain RW1. In in vitro assays, Fdx3 can be reduced by RedA2 (a reductase similar to clas s I cytochrome P-450 reductases), previously isolated from Sphingomonas sp. strain RW1. RedA2 exhibits a K-m value of 3.2 +/- 0.3 mu M for Fdx3. In vi vo coexpression of fdx3 and redA2 with dxnA1A2 confirmed that Fdx3 can serv e as an electron donor for the dioxin dioxygenase.