Regulation of transcription of genes required for fatty acid transport and unsaturated fatty acid biosynthesis in Escherichiacoli by FadR

Fatty acid biosynthesis and fatty acid degradation in Escherichiacoliare co‐ordinately regulated at the level of transcription by the product of the fadRgene, FadR. In the present work we investigate FadR interaction with the fabA and fadL promoters. The FadR‐responsive operator within fabA, O_A, was localized to a region −47 to −31 base pairs relative to the start of transcription using DNase I protection studies. The promoter and untranslated leader within fadL had two binding sites for FadR, O_L1 at −25 to −9 and O_L2 at −1 to +16 relative to the start of transcription. The binding affinity of FadR for O_A and O_L1 or O_L2 was lower than that for the single site within fadB (O_B) as measured using protein‐DNA gel retention assays. Overall, these experiments demonstrated that the affinity of FadR binding for DNA containing the fadB, fadL and fabA promoters was O_B > O_L1, O_L2 > O_A. We could not distinguish separate binding affinities for O_L1 or O_L2. We demonstrated repression of fadL transcription and activation of fabA transcription in vitro using run‐off transcription assays containing purified FadR and RNA polymerase.