Bio-probes that inhibit the action of auxin are useful tools for the study of auxin signaling. To screen for specific inhibitors of auxin signaling, we used an Arabidopsis transgenic line harboring the auxin-inducible promoter derived from PS-IAA4/5 and the reporter gene, GUS (beta-glucuronidase). In this transgenic plant, the exogenous auxin specifically enhanced the expression of the GUS reporter gene. A novel 22-membered spiroketal-macrolide, yokonolide A (1), and related previously known compound, A82548A (2), were isolated from Streptomyces diastatochromogenes B59 as inhibitors of auxin inducible gene expression. The absolute structure of I was determined by detailed spectral analyses and chemical derivatization. 1 and 2 completely inhibited the auxin-induced transcription of the reporter gene at 5 and 1 microm, respectively. In contrast, 1 and 2 did not affect the translation of GUS reporter transcripts. In addition, 1 and 2 did not inhibit the gibberellin-induced alpha-amylase expression at 100 microM in barley aleurone cells. These results suggest that 1 and 2 specifically inhibit auxin signaling leading to auxin-mediated gene expression.