Natural constitutive promoters from Actinoplanes siamensis TBRC 4237 enable high-level heterologous gene expression in Streptomyces coelicolor

Activation of cryptic biosynthetic gene clusters (BGCs) in actinomycetes often requires strong and reliable promoters, yet native promoters from rare actinomycetes remain scarce. Here, we used transcriptome analysis of Actinoplanes siamensis TBRC 4237 to identify highly expressed genes and selected 22 upstream regions as candidate constitutive promoters. Their activities were evaluated in Streptomyces coelicolor M1146 using indigoidine synthetase and latex-clearing protein (LCP) as heterologous reporters. Thirteen promoters drove indigoidine production at levels exceeding those of kasOp* and ermEp*, the widely used strong constitutive promoters in Streptomyces. Several promoters also yielded robust LCP activity, with promoter 650A achieving the highest expression across both reporter systems. Motif analysis showed no clear correlation between predicted -10/-35 elements and promoter strength, indicating additional regulatory features. These natural promoters provide valuable genetic parts for heterologous expression, BGC activation, and pathway refactoring in actinomycetes.