The alternative sigma factor sigmaH has been shown to play an important role in stress response and morphological differentiation in Streptomyces coelicolor. Its gene, sigH, is located in an operon with the gene encoding proposed anti-sigma factor UshX, and one of the promoters directing expression of the operon is dependent upon sigH. To clarify the function of S. coelicolor UshX, both the sigmaH and UshX proteins were overproduced in Escherichia coli and purified. In an in vitro transcription assay, sigmaH, after complementation with S. coelicolor core RNA polymerase, was able to recognize the sigH-dependent promoter, sigH-P2. This transcription was inhibited by UshX, if it was incubated with sigmaH prior to the addition of the core RNA polymerase. When sigmaH and UshX were incubated and electrophoresed through non-denaturing polyacrylamide gels, they formed a specific complex. These results showed that UshX is a specific anti-sigma factor for sigmaH, and the S. coelicolor sigH operon is directly autoregulated.