Conserved motifs within the mel2 locus and our working model. Conserved domains found within the Vibrio fisheri lux loci and Mycobacterium marinum mel2 locus (A) and proposed biochemical roles (B). Position of the M. marinum mel2 mutant transposon insertion is shown as a triangle above the mel2 locus (melF::Tn) and the structures of the complementing constructs, pJDC79 and pJDC75, are below the mel2 locus. The luxC-luxG region is located from the V. fisheri chromosome II and the luxH gene is from chromosome I (A). Conserved domains were identified using the NCBI conserved domain search. The designation for each conserved domain(s) is shown above each gene with an abbreviation for its proposed biochemical function in parenthesis. Abbreviations for proposed biochemical functions are as follows with additional conserved domains not shown in the figure in parenthesis: lux, luciferase; fer2, 2Fe-2S iron-sulfur cluster binding domain (cd00207); NAD, flavodoxin oxidoreductases and oxidoreductase NAD-binding domain (pfam00175); FADred, FAD-dependent oxidoreductases; hyd, abhydrolase alpha/beta hydrolase fold (pfam00561); acylT, predicted acyltransferases; FMNred, flavin reductase like domain; DHBPs, 3,4-dihydroxy-2-butanone 4-phosphate synthase; GTPcycII, GTP cyclohydrolase II; dehyd, dehydrogenase; acylS, acyl-protein synthetase. We constructed a hypothetical model for how these proteins might interact to reduce reactive oxygen species (B). The resulting pathways for mel2 are similar to the lux pathways, but are better adapted to serve as a potential defense against oxidative stress through the presence of an epoxide hydrolase (melH), in addition to the reduction of fatty acid aldehydes observed with lux. R represents a number of potential fatty acid molecules that could be used as substrates for these reactions. The LuxCDE proteins function as a complex to produce the aldehydes used to reduce oxygen by LuxAB and we have depicted a similar situation for MelGHK.