Transporter domains are predictive of siderophore BGCs. (A) The frequencies of common transporter Pfam domains across siderophore BGCs and BGCs of other known activities in Gram-positive and Gram-negative bacteria. Bars in green were significantly different in frequency between the two classes (Fisher's exact test; Q < 0.05) (B) Precision-recall curves for two-layer decision trees classifying siderophore BGCs using Pfam transporter, CATH transporter, and Pfam biosynthetic gene features in Gram-negative and Gram-positive bacteria. (C) Examples of three siderophore BGCs without activity labels in MIBiG 2.0, which could be identified using transporter frequencies. Transporter genes are blue, core biosynthetic genes (NRPS and PKS) are dark red, accessory biosynthetic genes are light red, and regulatory genes are green.