Escherichia coli FadR Positively Regulates Transcription of the fabB Fatty Acid Biosynthetic Gene

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Abstract
In Escherichia coli expression of the genes of fatty acid degradation (fad) is negatively regulated at the transcriptional level by FadR protein. In contrast the unsaturated fatty acid biosynthetic gene, fabA, is positively regulated by FadR. We report that fabB, a second unsaturated fatty acid biosynthetic gene, is also positively regulated by FadR. Genomic array studies that compared global transcriptional differences between wild-type and fadR-null mutant strains, as well as in cultures of each strain grown in the presence of exogenous oleic acid, indicated that expression of fabBwas regulated in a manner very similar to that of fabAexpression. A series of genetic and biochemical tests confirmed these observations. Strains containing both fabB andfadR mutant alleles were constructed and shown to exhibit synthetic lethal phenotypes, similar to those observed infabA fadR mutants. A fadR strain was hypersensitive to cerulenin, an antibiotic that at low concentrations specifically targets the FabB protein. A transcriptional fusion of chloramphenicol acetyltransferase (CAT) to the fabBpromoter produces lower levels of CAT protein in a strain lacking functional FadR. The ability of a putative FadR binding site within thefabB promoter to form a complex with purified FadR protein was determined by a gel mobility shift assay. These experiments demonstrate that expression of fabB is positively regulated by FadR.