molecules are Acyl-CoA or Malonyl-CoA but poyketide synthases can use multiple primers including acetyl-CoA, propionyl-CoA, isobutyryl-CoA, cyclohexanoyl-CoA, 3-amino-5-hydroxybenzoyl-CoA, or cinnamyl-CoA. In both fatty acid synthesis and polyketide synthesis these CoA carriers will be exchanged for ACP before they are incorporated into the growing molecule. During the elongation steps of fatty acid synthesis, ketosynthase, ketoreductase, dehydratase, and enoylreductase are all used in sequence to create a saturated fatty acid then postsynthetic modification can be done to create an unsaturated or cyclo fatty acid. However, in polyketide synthesis these enzymes can be used in different combinations to create segments of polyketide that are saturated, unsaturated, or have a hydroxyl or carbonyl functional group. There are also enzymes used in both fatty acid synthesis and polyketide synthesis that can make modifications to the molecule after it has been synthesized. As far as regulating the length of the molecule being synthesized, the specific mechanism by which fatty acid chain length remains unknown but it is expected that ACP-bound fatty acid chains of the correct length act as allosteric inhibitors of the fatty acid synthesis enzymes. In polyketide synthesis, the synthases are composed of modules in which the order of enzymatic reactions is defined by the structure of the protein complex. This means that once the molecule reaches the last reaction of the last module, the polyketide is released from the complex by a thioesterase enzyme. Therefore, regulation of fatty acid chain length is most likely due to allosteric regulation, and regulation of polyketide length is due to a specific enzyme within the polyketide synthase. Application Since the late 1980s and early 1990s research on polyketide synthases (PKS), a number of strategies for the genetic modification of such PKS have been developed and elucidated. Such changes in PKS are of particular interest to the pharmaceutical industry as new compounds with antibiotic or other antimicrobial effects are commonly synthesized after changes to the structure of the PKS have been made. Engineering the PKS complex is a much more practical method than synthesizing each product via chemical reactions in vitro due to the cost of reagents and the number of reactions that must take place. Just to exemplify the potential rewards of synthesizing new and effective antimicrobials, in 1995, the worldwide sales of erythromycin and its derivatives exceeded 3.5 billion dollars. This portion will examine the modifications of structure in the DEBS PKS to create new products