The razABCD regulatory island links azalomycin F biosynthesis in Streptomyces iranensis to orsellinic acid production in the fungus Aspergillus nidulans

Streptomyces iranensis induces the production of orsellinic acid and a range of its derivatives in the fungus Aspergillus nidulans. The arginoketide azalomycin F was identified as the bacterial signaling molecule that activates otherwise silent biosynthetic gene clusters (BGCs) in diverse fungi. To date, however, little is known about the regulatory mechanisms governing the azalomycin F BGC, beyond the requirement of the regulatory gene bldA. Here, building on the known role of bldA, we report the discovery of a regulatory island, razABCD, encoding four LuxR-family transcriptional regulators. Through the generation of deletion mutants and subsequent analysis of azalomycin F production, RazA, RazB, and RazD were identified as key regulators of the azalomycin F BGC. This regulatory island thereby governs the induction of the orsellinic acid BGC in A. nidulans. The DNA-binding motif of RazD (5'-CTAGGGAWTY-3') was defined using electrophoretic mobility shift assays (EMSAs) and surface plasmon resonance analyses with the purified DNA-binding domain of RazD. Molecular genetic analyses further revealed that razA and razB act hierarchically upstream of razD. Collectively, our findings establish a regulatory link between the control of a bacterial biosynthetic gene cluster and the activation of a silent fungal BGC, providing mechanistic insight into interkingdom metabolic communication.