There is a great demand for precisely quantitating the expression of genes of interest in synthetic and systems biotechnology as new and fascinating insights into the genetics of streptomycetes have come to light. Here, we developed, for the first time to our knowledge, a quantitative method based on flow cytometry and a superfolder green fluorescent protein (sfGFP) at single-cell resolution in Streptomyces. Single cells of filamentous bacteria were obtained by releasing the protoplasts from the mycelium, and the dead cells could be distinguished from the viable ones by propidium iodide (PI) staining. With this sophisticated quantitative method, some 200 native or synthetic promoters and 200 ribosomal binding sites (RBSs) were characterized in a high-throughput format. Furthermore, an insulator (RiboJ) was recruited to eliminate the interference between promoters and RBSs and improve the modularity of regulatory elements. Seven synthetic promoters with gradient strength were successfully applied in a proof-of-principle approach to activate and overproduce the cryptic lycopene in a predictable manner in Streptomyces avermitilis. Our work therefore presents a quantitative strategy and universal synthetic modular regulatory elements, which will facilitate the functional optimization of gene clusters and the drug discovery process in Streptomyces.synthetic biology | natural product | flow cytometry | single-cell resolution | modular regulatory elements S treptomycetes are well known as the most abundant source of bioactive secondary metabolites (1), including medically important antimicrobial agents [e.g., chloramphenicol from Streptomyces venezuelae (2)], agricultural chemicals [e.g., avermectin from Streptomyces avermitilis (3)], and anticancer agents and immunosuppressants [e.g., rapamycin from Streptomyces hygroscopicus (4)]. However, the increasing difficulty of discovering novel drugs via traditional high-throughput screening and the "one strain many compounds" approach is frustrating pharmaceutical productivity (5, 6). Deciphering the genome sequences of Streptomyces surprisingly established the presence of a plethora of gene clusters encoding for yet-unobserved molecules, even in intensively investigated Streptomyces coelicolor A3 (2), revealing a much higher potential of novel bioactive agent production than originally anticipated (7,8). Therefore, the enormous number of natural products that have been obtained likely represent only a tiny portion of the repertoire of bioactive compounds that can possibly be produced. This has brought about extensive research into applied genomics aimed at investigating these new gene clusters, generally referred to as "cryptic," "silent," or "orphan" (9-11). With data on more than 12,000 in-house draft bacterial genomes, the potential for the discovery of a number of novel chemicals encrypted in silent biosynthetic gene clusters has been detected by genome mining.Many new strategies have been documented for "awakening" poorly expressed and/or silent gene clusters in Strept...

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