New approaches to antimicrobial discovery are needed to address the growing threat of antibiotic resistance. The Streptomyces genus, a proven source of antibiotics, is recognized as having a large reservoir of untapped secondary metabolic genes, many of which are likely to produce uncharacterized compounds. However, most of these compounds are currently inaccessible, as they are not expressed under standard laboratory conditions. Here, we present a novel methodology for activating these "cryptic" metabolites by heterologously expressing a constitutively active pleiotropic regulator. By screening wild Streptomyces isolates, we identified the antibiotic siamycin-I, a lasso peptide that we show is active against multidrug pathogens. We further revealed that siamycin-I interferes with cell wall integrity via lipid II. This new technology has the potential to be broadly applied for use in the discovery of additional "cryptic" metabolites.