<jats:title>Summary</jats:title><jats:p>A protein binding to the <jats:styled-content>a</jats:styled-content>uto<jats:styled-content>r</jats:styled-content>egulatory <jats:styled-content>e</jats:styled-content>lement (ARE) upstream of the regulatory <jats:italic>ccaR</jats:italic> gene of <jats:italic>Streptomyces clavuligerus</jats:italic> was isolated previously by DNA affinity binding. The <jats:italic>areB</jats:italic> gene, encoding this protein, is located upstream and in opposite orientation to the <jats:italic>leuCD</jats:italic> operon of <jats:italic>S. clavuligerus</jats:italic>; it encodes a 239‐amino‐acid protein of the IclR family with a helix–turn–helix motif at the N‐terminal region. An <jats:italic>areB</jats:italic>‐deleted mutant, <jats:italic>S. clavuligerus</jats:italic>Δ<jats:italic>areB</jats:italic>, has been constructed by gene replacement. This strain requires leucine for optimal growth in defined media. Expression of the <jats:italic>leuCD</jats:italic> operon is retarded in <jats:italic>S. clavuligerus</jats:italic>Δ<jats:italic>areB</jats:italic>, because AreB binds the <jats:italic>areB‐leuCD</jats:italic> intergenic region acting as a positive modulator. Clavulanic acid and cephamycin C production are improved in the Δ<jats:italic>areB</jats:italic> mutant although no drastic difference in <jats:italic>ccaR</jats:italic> expression was observed. Pure recombinant AreB protein does not bind the ARE<jats:sub><jats:italic>ccaR</jats:italic></jats:sub> sequence (as shown by EMSA) unless filtered extracts from <jats:italic>S. clavuligerus</jats:italic> ATCC 27064‐containing molecules of Mr lower than 10 kDa are added to the binding reaction. Restoration of binding to the ARE<jats:sub><jats:italic>ccaR</jats:italic></jats:sub> sequence is not observed when filtered extracts are obtained from the Δ<jats:italic>areB</jats:italic> mutant, suggesting that biosynthesis of the small‐molecular‐weight effector is also controlled by AreB.