Abstract
The loops of modular polyketide synthases (PKSs) serve diverse functions
but are largely uncharacterized. They frequently contain amino acid
repeats resulting from genetic events such as slipped-strand mispairing.
Determining the tolerance of loops to amino acid changes would aid in
understanding and engineering these multidomain molecule factories.
Here, tandem repeats in the DNA encoding 949 modules within 129
cis-acyltransferase PKSs were catalogued, and the locations of the
corresponding amino acids within the module were identified. The most
frequently inserted interdomain loop corresponds with the updated module
boundary immediately downstream of the ketosynthase (KS), while the
loops bordering the dehydratase (DH) were nearly intolerant to such
insertions. An analysis of the loops bordering the acyl carrier protein
(ACP) reveals they are relatively short (14±6 residues), that they
resist large increases in length, and that ACP may rely on
acyltransferase (AT) accessing a conformation like that observed through
electron microscopy of the pikromycin PKS. From the 949 modules, no
repetitive sequence loop insertions are located within ACP, and only 2
reside within KS, indicating the sensitivity of these domains to
alteration.