Characterizing and Engineering Rhamnose-Inducible Regulatory Systems for Dynamic Control of Metabolic Pathways in Streptomyces.

Fine-tuning gene expression is of great interest for synthetic biotechnological applications. This is particularly true for the genus Streptomyces, which is well-known as a prolific producer of diverse natural products. Currently, there is an increasing demand to develop effective gene induction systems. In this study, bioinformatic analysis revealed a putative rhamnose catabolic pathway in multiple Streptomyces species, and the removal of the pathway in the model organism Streptomyces coelicolor impaired its growth on minimal media with rhamnose as the sole carbon source. To unravel the regulatory mechanism of RhaR, a LacI family transcriptional regulator of the catabolic pathway, electrophoretic mobility shift assays (EMSAs) were performed to identify potential target promoters. Multiple sequence alignments retrieved a consensus sequence of the RhaR operator (rhaO). A synthetic biology-based strategy was then deployed to build rhamnose-inducible regulatory systems, referred to as rhaRS1 and rhaRS2, by assembling the repressor/operator pair RhaR/rhaO with the well-defined constitutive kasO* promoter. Both rhaRS1 and rhaRS2 exhibited a high level of induced reporter activity, with no leaky expression. rhaRS2 has been proven successful for the programmable production of actinorhodin and violacein in Streptomyces. Our study expanded the toolkit of inducible regulatory systems that will be broadly applicable to many other Streptomyces species.