Regulation of fatty acid degradation in Escherichia coli: fadR superrepressor mutants are unable to utilize fatty acids as the sole carbon source.

Localized mutagenesis of the fadR region of the Escherichia coli chromosome resulted in the isolation of two classes of fadR regulatory mutants. The first class was constitutive for the fatty acid degradative enzymes and presumably defective for fadR function. The second class was rarer and resulted in the inability to utilize fatty acids as a sole carbon source (Fad-). These fadR superrepressor mutants [fadR(S)] had greatly reduced levels of the beta-oxidative enzymes required for growth on fatty acids. The fadR(S) mutants reverted to Fad+ at a high frequency (10(-5], and the resulting Fad+ revertants were constitutive for expression of the fad enzymes (fadR). Merodiploid analysis showed the fadR(S) allele to be dominant to both fadR+ and fadR alleles.