The bacterium Mycobacterium marinum, has a larger genome compared to the causative agents of human tuberculosis (Mycobacterium tuberculosis) and Buruli ulcer (Mycobacterium ulcerans) which have undergone substantial reductive evolution.1 The genome of M. marinum contains an increased number of cytochrome P450 genes (47) and an expanded electron transfer complement.2-3 Twelve ferredoxin genes are associated with the CYPome and the majority of these are uncharacterised ferredoxins of the 3/4Fe-4S type. In their iron-sulfur cluster binding motif (CXX?XXC(X)nCP), these ferredoxins have non-standard residues at the ? position of the sequence.4 Instead of the cysteine residue expected of a [4Fe-4S] ferredoxin, they contain histidine, asparagine, tyrosine, serine, threonine and phenylalanine residues. A number of these have been successfully purified both aerobically and anaerobically, and preliminary studies have been used to characterize the identity of the metal-ion cluster of these proteins. Activity data from P450s coupled with wild-type and single nucleotide ferredoxin mutants suggest the non-standard residue plays a role in protein stability or electron transfer. In the course of this work, several P450s from previously unstudied families have been characterized by UV spectroscopy and X-ray crystallography and their native electron transfer chain reconstituted.