Gamma-butyrolactone autoregulator receptors of the genus Streptomyces have a common activity as DNA-binding transcriptional repressors, controlling secondary metabolism and/or morphological differentiation. A gene encoding a gamma-butyrolactone autoregulator receptor was cloned from a bafilomycin B1 producer, Kitasatospora setae, for the first time from a non-Streptomyces genus of actinomycetes, and its function was evaluated by in vitro and in vivo analyses. The gene fragment was initially cloned by PCR with primers designed from two highly conserved regions of Streptomyces autoregulator receptors (BarA, FarA, ScbR, and ArpA), followed by genomic Southern hybridization yielding a 7-kb BamHI fragment on which a 654-bp receptor gene (ksbA) was identified. The recombinant KsbA protein demonstrated clear binding activity toward 3H-labeled autoregulators, especially toward [3H]SCB1, confirming that ksbA encodes a real autoregulator receptor of K. setae. To clarify the in vivo function of ksbA, a ksbA-disrupted strain was constructed by means of homologous recombination after introducing a ksbA disruption construct via transconjugation from Escherichia coli. No difference in morphology was found between the wild-type strain and the ksbA disruptants. However, the ksbA disruptants started producing bafilomycin 18 h earlier than the wild-type strain and showed a 2.4-fold-higher accumulation of bafilomycin. The phenotype was restored to the original wild-type phenotype by complementation with intact ksbA, indicating that the autoregulator receptor protein of K. setae acts as a primary negative regulator of the biosynthesis of bafilomycin but plays no role in cytodifferentiation of K. setae. This indicates that, unlike the A-factor receptor of Streptomyces griseus, the autoregulator receptor (ksbA) of K. setae belongs to a family of autoregulator receptors which control secondary metabolism but play no role in morphological differentiation.