Analysis of the nearly identical morpholine monooxygenase-encoding mor genes from different Mycobacterium strains and characterization of the specific NADH : ferredoxin oxidoreductase of this cytochrome P450 system

Abstract

Cloning and sequencing of the morABC operon region revealed the genes encoding the three components of a cytochrome P450 monooxygenase, which is required for the degradation of the N-heterocycle morpholine by Mycobacterium sp. strain HE5. The cytochrome P450 (P450(mor)) and the Fe(3)S(4) ferredoxin (Fd(mor)), encoded by morA and morB, respectively, have been characterized previously, whereas no evidence has hitherto been obtained for a specifically morpholine-induced reductase, which would be required to support the activity of the P450(mor) system. Analysis of the mor operon has now revealed the gene morC, encoding the ferredoxin reductase of this morpholine monooxygenase. The genes morA, morB and morC were identical to the corresponding genes from Mycobacterium sp. strain RP1. Almost identical mor genes in Mycobacterium chlorophenolicum PCP-1, in addition to an inducible cytochrome P450, pointing to horizontal gene transfer, were now identified. No evidence for a circular or linear plasmid was found in Mycobacterium sp. strain HE5. Analysis of the downstream sequences of morC revealed differences in this gene region between Mycobacterium sp. strain HE5 and Mycobacterium sp. strain RP1 on the one hand, and M. chlorophenolicum on the other hand, indicating insertions or deletions after recombination. Downstream of the mor genes, the gene orf1', encoding a putative glutamine synthetase, was identified in all studied strains. The gene morC of Mycobacterium sp. strain HE5 was heterologously expressed. The purified recombinant protein FdR(mor) was characterized as a monomeric 44 kDa protein, being a strictly NADH-dependent, FAD-containing reductase. The K(m) values of FdR(mor) for the substrate NADH (37.7 +/- 4.1 microM) and the artificial electron acceptors potassium ferricyanide (14.2 +/- 1.1 microM) and cytochrome c (28.0 +/- 3.6 microM) were measured. FdR(mor) was shown to interact functionally with its natural redox partner, the Fe(3)S(4) protein Fd(mor), and with the Fe(2)S(2) protein adrenodoxin, albeit with a much lower efficiency, but not with spinach ferredoxin. In contrast, adrenodoxin reductase, the natural redox partner of adrenodoxin, could not use Fd(mor) in activity assays. These results indicated that FdR(mor) can utilize different ferredoxins, but that Fd(mor) requires the specific NADH : ferredoxin oxidoreductase FdR(mor) from the P450(mor) system for efficient catalytic function.