Intra- and inter-biosynthetic gene cluster allelic variation as drivers of chemical diversification in <i>Streptomyces</i>

In this essay, we postulate that biosynthetic gene clusters (BGCs) chemical diversity arises in Streptomyces cultures from allelic variation both within a single BGC-intra-BGC, and across homologous BGCs-inter-BGC. These layers of genetic diversification reshape pathway architecture, modulate catalytic efficiency, and redefine substrate channeling, ultimately expanding the repertoire of specialized metabolites produced by Streptomyces. On one hand, previous metabolic analyses enabled by the one strain-many compounds framework have provided a comprehensive view of such chemical diversity, while enzyme promiscuity has emerged as a key mechanistic driver contributing to this chemical diversification generating expanded suites of structural analogs. On the other hand, large comparative genomics datasets and phylogenomics have revealed recurrent patterns of enzyme modular rearrangement, domain-level substitutions, and regulatory rewiring that underpin vertical metabolite divergence. Together, the interplay of BGC allelic heterogeneity, pathway modularity, and catalytic promiscuity forms a multilayered evolutionary strategy that fuels the emergence of novel chemical space-derived from a single BGC-in Streptomyces cultures.