A NOVEL [2FE-2S] FERREDOXIN FROM PSEUDOMONAS-PUTIDA MT2 PROMOTES THE REDUCTIVE REACTIVATION OF CATECHOL 2,3-DIOXYGENASE

Catechol a,3-dioxygenase (XS LE) is a component of the TOL plasmid-enc oded pathway for the degradation of toluene and xylenes and catalyzes the dioxygenolytic cleavage of the aromatic ring. Purified XylE is oxy gen-sensitive and unstable in vitro, particularly in the presence of s ubstituted catechol substrates, but it is stabilized in vivo by anothe r protein, XylT, encoded by the xylT gene located just upstream of xyl E, In this study, we have purified to homogeneity the XylT product fro m a recombinant Escherichia coil strain containing a hyper-expressible xylT gene and characterized it as a novel [2Fe-2S] ferredoxin, It is the first example of a soluble ferredoxin with a net positive charge a t neutral pH, The EPR signal of the iron sulfur cluster has rhombic sy mmetry as is the case for plant-type ferredoxins, but the XylT absorba nce spectrum resembles more closely that of adrenodoxin, The midpoint redox potential was determined to be -373 +/- 6 mV, at pH 8.5, XylT wa s unusually unstable for a [2Fe-2S] ferredoxin, with half-lives of 69 min at 25 degrees C in air and 70 min at 37 degrees C in argon, With p hotochemically reduced 5-deazaflavin for the controlled generation of reductant, it was demonstrated that XylT mediates the rapid reactivati on of purified inactive catechol 2,3-dioxygenase in vitro, Inactivatio n of XylE by 4-methylcatechol resulted in oxidation of the active site iron to a high spin ferric state that was detectable by EPR, Spectros copic evidence presented here demonstrates that XylT reactivates XylE through reduction of the iron atom in the active site of the enzyme. I t is the first instance of a ferredoxin-mediated reactivation of an en zyme. The level of expression of XylT in Pseudomonas putida mt2 cells is low and the calculated XylT/XylE molar ratio is consistent with the proposal that XylE reactivation involves catalytic nonstoichiometric amounts of XylT.