and pcp domain that is associated with 2,4-diaminobutryic acid. this continues with each amino acid until the linear peptide chain is completed. the last module will have a thioesterase to complete the cyclization and form the product colistin. the loading domain 6-methylheptanoic acid is shown in salmon; yellow is 2,4-diaminobutryic acid; light blue is threonine; magenta is leucine.