Abstract
Adenylation domains are the main contributor to structural complexity among nonribosomal peptides due to their varied but stringent substrate selection. Several in vitro assays to determine the substrate specificity of these dedicated biocatalysts have been implemented, but high sensitivity is often accompanied by the cost of laborious procedures, expensive reagents or the requirement for auxiliary enzymes. Here, we describe a simple protocol that is based on the removal of ferric iron from a preformed chromogenic complex between ferric iron and Chrome Azurol S. Adenylation activity can be rapidly followed by a decrease in absorbance at 630 nm, visualized by a prominent color change from blue to orange.