Substrate specificity of a ketosynthase domain involved in bacillaene biosynthesis

• Zhiyong Yin and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2024, 20, 734–740, doi:10.3762/bjoc.20.67

• (PKS). The type I of these enzymes are megasynthases composed of several catalytically active domains that can either act iteratively with the same set of domains catalysing the incorporation of several extender units into a growing polyketide chain, or non-iteratively with one set of domains acting

• compound is made through a trans-AT polyketide synthase–non-ribosomal peptide synthase (PKS-NRPS) hybrid [9][10]. Instead of using the classical domain organisation KS-AT-ACP with AT domains integrated into the PKS, trans-AT PKSs utilize discrete ATs that are not an integral part of the PKS, but rather

• cooperate with the PKS “in trans” [11][12]. Notably, in B. subtilis the giant bacillaene biosynthesis machinery forms an organelle-like complex that can be observed through cryoelectron microscopy [13]. The structure elucidation of “bacillaene” through extensive NMR spectroscopic methods revealed the

Chemoenzymatic synthesis of macrocyclic peptides and polyketides via thioesterase-catalyzed macrocyclization

• Senze Qiao,
• Zhongyu Cheng and
• Fuzhuo Li

Beilstein J. Org. Chem. 2024, 20, 721–733, doi:10.3762/bjoc.20.66

• and multifunctional enzymatic assembly, nonribosomal peptide synthases (NRPS), polyketide synthases (PKS), and hybrid NRPS/PKS systems, which are organized into sets of functional domains known as modules and function through a similar mechanism [9][10][11][12]. Each NRPS module is composed of three

• essential domains, namely adenylation (A), condensation (C), and peptidyl carrier protein (PCP). Each type I PKS module consists of three core domains containing acyltransferase (AT), ketosynthase (KS), and acyl carrier protein (ACP). PCP and ACP are collectively called thiolation domain (T). The sequence

• responsible for the structural diversity of natural products, both NRPS and PKS contain thioesterase (TE) domains in the final elongation module, which contribute to terminating biosynthesis [13][14]. Typically, TE domains cleave the thioester bond between the last PCP or ACP domain and the intermediate of

New variochelins from soil-isolated Variovorax sp. H002

• Jabal Rahmat Haedar,
• Aya Yoshimura and
• Toshiyuki Wakimoto

Beilstein J. Org. Chem. 2024, 20, 692–700, doi:10.3762/bjoc.20.63

• draft genome sequence of the H002 strain identified the variochelin biosynthetic gene cluster (var), which encodes PKS (polyketide synthase) and NRPS (non-ribosomal peptide synthetase) genes. Finally, the siderophores isolated in this study exhibited antibacterial activity against several bacteria

• , we also identified a var gene cluster containing NRPS and PKS genes: the domain organizations of NRPS and PKS, and the adjacently encoded modification enzymes, were comparable to those of the gene cluster reported by Nett et al. with 92–99% identity at the protein level [5] (Figure 3a and Figure S42

• in Supporting Information File 1). To investigate whether the identified var biosynthetic gene cluster is responsible for variochelin production, we substituted varG encoding the polyketide synthase (PKS) module with a chloramphenicol resistance (cmR) gene cassette, using homologous recombination

Chemical and biosynthetic potential of Penicillium shentong XL-F41

• Ran Zou,
• Xin Li,
• Xiaochen Chen,
• Yue-Wei Guo and
• Baofu Xu

Beilstein J. Org. Chem. 2024, 20, 597–606, doi:10.3762/bjoc.20.52

• version of antiSMASH 7.0 [25] software for the analysis of the Penicillium shentong XL-F41 genome, we identified 46 BGCs. These include 13 NRPS-like fragments, 6 NRPS, 13 type I PKS, 2 PKS/NRPS hybrids, 1 NI-siderophore, 2 NRP-metallophore/NRPS hybrids, 1 fungal RiPP with POP or UstH peptidase types, 1

• fungal-RiPP-like/T1PKS, 1 betalactone, 1 PKS type I/NRPS/indole hybrid, 1 fungal-RiPP-like/T1PKS hybrid, 1 NRP-metallophore/NRPS hybrid, NRPS-like/terpene/phosphonate hybrids, 3 terpenes, and 1 indole-related cluster (Table 5). BGC 7.3, identified as an indole-type gene cluster, includes genes for

Recent developments in the engineered biosynthesis of fungal meroterpenoids

• Zhiyang Quan and
• Takayoshi Awakawa

Beilstein J. Org. Chem. 2024, 20, 578–588, doi:10.3762/bjoc.20.50

• metabolic genes for ease of gene transfer and high substance production capabilities [10][11]. The expression of trt4 (polyketide synthase, PKS), trt2 (prenyltransferase, PT), trt5 (methyltransferase, MT), trt8 (flavin-dependent monooxygenase, FMO), and trt1 (meroterpenoid cyclase, CYC) in A. oryzae NSAR1

• , the combinatorial biosynthesis of diterpene pyrones, derived from a pyrone skeleton and a C20 terpenoid skeleton, has been achieved by combining multiple biosynthetic pathways in a fungal heterologous expression host [16]. First, subA (PKS), subC (PT), subD (GGPP synthase), subE (FMO), and subB (CYC

• pathways Furthermore, there are examples of the enzymatic synthesis of plant-derived pharmaceutical meroterpenoids through the heterologous expression of a combination of fungal and plant biosynthetic enzymes. The biosynthetic enzymes StbA (polyketide synthase, PKS) and StbC (prenyltransferase, PT) from

Identification of the p-coumaric acid biosynthetic gene cluster in Kutzneria albida: insights into the diazotization-dependent deamination pathway

• Seiji Kawai,
• Akito Yamada,
• Yohei Katsuyama and
• Yasuo Ohnishi

Beilstein J. Org. Chem. 2024, 20, 1–11, doi:10.3762/bjoc.20.1

• biosynthetic pathway [13]. In this pathway, 3-amino-4-hydroxybenzoic acid (3,4-AHBA, 1), synthesized by AvaH and AvaI, is loaded onto AvaA3 (carrier protein) by AvaA1 (AMP-dependent ligase), resulting in 3,4-AHBA-AvaA3. A highly reducing type II polyketide synthase (PKS) system [15][16] (AvaA2, A4, A5, and A8

• , indicating that CmaG is involved in the production of 6, although it is not essential (Figure 2B). In addition, the ΔcmaG strain produced a higher amount of compound 9 than S. albus-cma. Because compound 9 seems to be a shunt product derived from 3,4-AHBA (1), which is the starter substrate of Cma PKS, this

• result indicated that CmaG is a component of Cma PKS. From NMR analysis and high-resolution (HR)MS analysis ([M + H]+ ion at m/z = 222.0766, which corresponds to C11H12NO4+, calcd. 222.0761) of purified 6, the structure of 6 was determined as N-acetyl-3-aminocoumaric acid (Figures S9–S13 and Table S1 in

Secondary metabolites of Diaporthe cameroonensis, isolated from the Cameroonian medicinal plant Trema guineensis

• Bel Youssouf G. Mountessou,
• Élodie Gisèle M. Anoumedem,
• Blondelle M. Kemkuignou,
• Yasmina Marin-Felix,
• Frank Surup,
• Marc Stadler and
• Simeon F. Kouam

Beilstein J. Org. Chem. 2023, 19, 1555–1561, doi:10.3762/bjoc.19.112

• great structural variability such as polyketides, terpenoids, polyketide synthase–nonribosomal peptide synthetase (PKS–NRPS) alkaloids, and cytochalasins, which have been considered as taxonomic markers of the genus [7][8][9][10]. However, it is worthwhile to mention that the name Phomopsis should no

Navigating and expanding the roadmap of natural product genome mining tools

• Friederike Biermann,
• Sebastian L. Wenski and
• Eric J. N. Helfrich

Beilstein J. Org. Chem. 2022, 18, 1656–1671, doi:10.3762/bjoc.18.178

• combination of all genome mining approaches will pave the way towards a more comprehensive understanding of the full biosynthetic repertoire encoded in microbial genome sequences. Keywords: genome mining; natural product biosynthesis; non-canonical pathways; PKS; NRPS; RiPP; Introduction In 2002, the genome

• [26]), acyltransferase (AT) (for cis-AT PKS [15]), or ketosynthase (KS) domains (in trans-acyltransferase PKS systems [19][27]). Moreover, in the large majority of cases, the gene order within a BGC reflects the order of the corresponding enzymes during the biosynthesis of the associated NP [19

• ]. trans-AT PKSs are much more complex than cis-AT PKS systems as they harbor non-elongating modules, cryptic domains and seemingly superfluous domains. Moreover, they frequently employ a number of trans-acting modifying enzymes, are characterized by modules that are split between proteins and they often

Synthesis of tryptophan-dehydrobutyrine diketopiperazine and biological activity of hangtaimycin and its co-metabolites

• Houchao Xu,
• Anne Wochele,
• Minghe Luo,
• Gregor Schnakenburg,
• Yuhui Sun,
• Heike Brötz-Oesterhelt and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2022, 18, 1159–1165, doi:10.3762/bjoc.18.120

• -AT, [3][4]) polyketide synthase (PKS) and non-ribosomal peptide synthase (NRPS) [2] with a dehydrating bimodule [5][6] involved in the installation of the remaining Z-configured double bond within the polyketide backbone [7]. Furthermore, a cytochrome P450 monooxygenase was recently shown to be

Bioinspired tetraamino-bisthiourea chiral macrocycles in catalyzing decarboxylative Mannich reactions

• Hao Guo,
• Yu-Fei Ao,
• De-Xian Wang and
• Qi-Qiang Wang

Beilstein J. Org. Chem. 2022, 18, 486–496, doi:10.3762/bjoc.18.51

• , we believe this type of biomimetic chiral macrocycles will find more applications as catalysts in other reactions. Design of PKS-inspired multifunctional amino-thiourea macrocycle catalysts. Proposed catalytic mechanism. Synthesis of tetraamino-bisthiourea chiral macrocycles M1–M12. The synthesis of

Secondary metabolites of Bacillus subtilis impact the assembly of soil-derived semisynthetic bacterial communities

• Heiko T. Kiesewalter,
• Carlos N. Lozano-Andrade,
• Mikael L. Strube and
• Ákos T. Kovács

Beilstein J. Org. Chem. 2020, 16, 2983–2998, doi:10.3762/bjoc.16.248

• synthesised by NRPS gene clusters (surfactin, plipastatin, and bacillibactin) and one by a hybrid NRPS–PKS gene cluster (bacillaene, Figure 1). The well-studied biosurfactant surfactin, encoded by the srfAA-AD gene cluster, reduces the surface tension needed for swarming and sliding motility [38][39]. The

Fabclavine diversity in Xenorhabdus bacteria

• Sebastian L. Wenski,
• Harun Cimen,
• Natalie Berghaus,
• Sebastian W. Fuchs,
• Selcuk Hazir and
• Helge B. Bode

Beilstein J. Org. Chem. 2020, 16, 956–965, doi:10.3762/bjoc.16.84

• ), and polyketide synthases (PKS). Selected Xenorhabdus and Photorhabdus mutant strains were generated applying a chemically inducible promoter in front of the suggested fabclavine (fcl) biosynthesis gene cluster (BGC), followed by the analysis of the occurring fabclavines. Subsequently, known and

• of fabclavines as major antibiotics in several entomopathogenic strains, our work lays the foundation for the rapid fabclavine identification and dereplication as the basis for future work of this widespread and bioactive SM class. Keywords: antibiotic; fabclavine; NRPS-PKS hybrid; secondary

• ]. Fabclavines are hexapeptide/polyketide hybrids derived from nonribosomal peptide synthetases (NRPS) and a polyketide synthase (PKS), which are connected to an unusual polyamine derived from polyunsaturated fatty acid (PUFA) synthases [20]. Beside full-length fabclavines, also shortened derivatives were

Towards the total synthesis of chondrochloren A: synthesis of the (Z)-enamide fragment

• Jan Geldsetzer and
• Markus Kalesse

Beilstein J. Org. Chem. 2020, 16, 670–673, doi:10.3762/bjoc.16.64

• (Cmc5) by the groups of Höfle and Reichenbach in 2003 [12]. This PKS/NRPS-derived natural product shows only weak antibiotic effects in agar diffusion tests against Micrococcus luteus, Schizosaccharomyces pombe, Bacillus subtilis and Staphylococcus aureus [12]. The relative and absolute stereochemistry

Chemical synthesis of tripeptide thioesters for the biotechnological incorporation into the myxobacterial secondary metabolite argyrin via mutasynthesis

• David C. B. Siebert,
• Roman Sommer,
• Domen Pogorevc,
• Michael Hoffmann,
• Silke C. Wenzel,
• Rolf Müller and
• Alexander Titz

Beilstein J. Org. Chem. 2019, 15, 2922–2929, doi:10.3762/bjoc.15.286

• . aeruginosa [18]. Biotechnological engineering of producer strains aims to shutdown the natural substrate production and thereby increase the usually poor yields of the mutasynthesis products [19][20]. For bacterial natural products that originate from a polyketide synthase (PKS) or a nonribosomal peptide

Bacterial terpene biosynthesis: challenges and opportunities for pathway engineering

• Eric J. N. Helfrich,
• Geng-Min Lin,
• Christopher A. Voigt and
• Jon Clardy

Beilstein J. Org. Chem. 2019, 15, 2889–2906, doi:10.3762/bjoc.15.283

• thiotemplated assembly lines, such as type I polyketide synthases (PKS) and nonribosomal peptide synthetases (NRPS), are modular, with each module contributing a distinct fragment to the final product’s core structure – a short-chain carboxylic acid (PKS) or an amino acid (NRPS). The modularly defined template

• allow for quantitative control of product outcomes, while PKS and NRPS pathways feature qualitative control [40]. The gibberellins (e.g., gibberellin A4 (5), Figure 1) are an extreme example illustrating the promiscuity of terpene biosynthetic pathways with more than 130 different family members

• assist in refining our understanding of bacterial terpenoid biosynthesis. Examples of bioactive terpenoids. Repetitive electrophilic and nucleophilic functionalities in terpene and type II PKS-derived polyketide biosynthesis. a) Schematic representation. b) Type II PKS-derived polyketide biosynthesis. c

Nanangenines: drimane sesquiterpenoids as the dominant metabolite cohort of a novel Australian fungus, Aspergillus nanangensis

• Heather J. Lacey,
• Cameron L. M. Gilchrist,
• Andrew Crombie,
• John A. Kalaitzis,
• Daniel Vuong,
• Peter J. Rutledge,
• Peter Turner,
• John I. Pitt,
• Ernest Lacey,
• Yit-Heng Chooi and
• Andrew M. Piggott

Beilstein J. Org. Chem. 2019, 15, 2631–2643, doi:10.3762/bjoc.15.256

• side chains present in the (iso)nanangenines could be derived either from a fatty acid synthase (FAS) or polyketide synthase (PKS). For example, in aflatoxin biosynthesis, the hexanoyl started unit is supplied by a FAS [31], while in the meroterpenoid fumagillin biosynthesis, the unsaturated acyl chain

• is synthesised by a PKS [32]. The A. nanangensis genome was sequenced and a draft assembly of the genome was generated. A local BLASTp search of the A. nanangensis genome using the drimane synthase AstC as query returned a hit on scaffold 3, FE257_006542, which was immediately flanked by a highly

• -reducing PKS (FE257_006541) and a FAD-binding oxidoreductase (FE257_006543). AstC, though annotated as a HAD-like hydrolase and having low sequence homology to other characterised terpene synthases, contains sequence motifs conserved across both class I (DDxxD/E) and class II (DxDD, QW) terpene synthases

Isolation and biosynthesis of an unsaturated fatty acid with unusual methylation pattern from a coral-associated bacterium Microbulbifer sp.

• Amit Raj Sharma,
• Enjuro Harunari,
• Tao Zhou,
• Agus Trianto and
• Yasuhiro Igarashi

Beilstein J. Org. Chem. 2019, 15, 2327–2332, doi:10.3762/bjoc.15.225

• intermediate, from which deprotonation occurs at C9 to give an internal olefin (Scheme 1). In the case of 1, methylation at the C3 carbon is inconsistent with the regular methylation pattern that occurs in fatty acids synthesized by the FAS (fatty acid synthase) or polyketides from the PKS (polyketide synthase

Volatiles from the hypoxylaceous fungi Hypoxylon griseobrunneum and Hypoxylon macrocarpum

• Jan Rinkel,
• Alexander Babczyk,
• Tao Wang,
• Marc Stadler and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2018, 14, 2974–2990, doi:10.3762/bjoc.14.277

• (methyl-2H3)methionine. While the methylation pattern of the alternative structure 24c is difficult to understand via a polyketide biosynthesis mechanism, the formation of the assigned structure of 24 by a polyketide synthase (PKS) can be easily rationalised (Scheme 2). The acetate starter unit, bound to

• the acyl carrier protein (ACP) of an iterative fungal PKS, can be elongated with malonyl-SCoA (mal-SCoA) followed by C-methylation with S-adenosyl-L-methionine (SAM). Two more rounds of elongation with mal-SCoA, the first extension with C-methylation and action of a ketoreductase (KR), result in a

• compared to the anisoles. Compound 23 was recently reported from Euphorbia golondrina [36], but was never observed as a fungal natural product so far. Biosynthetically, the identified compound 23 can arise by a similar mechanism as discussed for 24, potentially as a minor product of the same PKS, only the

Volatiles from the tropical ascomycete Daldinia clavata (Hypoxylaceae, Xylariales)

• Tao Wang,
• Kathrin I. Mohr,
• Marc Stadler and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2018, 14, 135–147, doi:10.3762/bjoc.14.9

• is shown in Scheme 7 that is likely performed by a typical fungal iterative polyketide synthase (PKS). Starting from acyl-carrier-protein (ACP) bound acetate a first elongation step with malonyl-SCoA (Mal-SCoA) catalysed by an acyl transferase (AT) and a ketosynthase (KS) domain yields acetoacetyl

• -SACP. This may be followed by SAM-dependent C-methylation by a methyl transferase domain (MT). The stereochemical course for this reaction can be inferred from the 4R-configuration of the final product 11a, if indeed an iterative PKS is involved that should have the same stereochemical course for the

• Nodulisporium spp. (shown in the box in Scheme 7) [28][39] that may be formed by a similar PKS. Further investigations are required to identify the PKSs for this family of metabolites and to confirm the hypothetical biosynthesis as shown in Scheme 7. The volatile 9, a compound emitted in small amounts by D

Sulfation and amidinohydrolysis in the biosynthesis of giant linear polyenes

• Hui Hong,
• Markiyan Samborskyy,
• Katsiaryna Usachova,
• Katharina Schnatz and
• Peter F. Leadlay

Beilstein J. Org. Chem. 2017, 13, 2408–2415, doi:10.3762/bjoc.13.238

• DSM4137 (formerly Streptomyces violaceusniger DSM4137) [6] and it co-occurs with the closely-related antifungal mediomycins A (1c) and B (1d, Scheme 1) in Streptomyces mediocidicus ATCC 23936 [7]. Mediomycins are also produced by Streptomyces blastmyceticus [8]. The genes for the assembly-line PKS for

• are summarised in Table S4 (Supporting Information File 1). Both gene clusters encode a giant modular PKS housing 27 extension modules, distributed across nine PKS subunits (Scheme 2). This is exactly the number of extension modules predicted on the basis of the known structures of 1a–d. Detailed

• comparison of our amino acid sequences for the Cle PKS [10] and the Med PKS with the sequence recently reported for the Med PKS of S. blastmyceticus [8] showed in each case a high degree of amino acid sequence identity across the entire PKS (≈96% identity) and essentially perfect conservation of sequence in

Strategies in megasynthase engineering – fatty acid synthases (FAS) as model proteins

• Manuel Fischer and
• Martin Grininger

Beilstein J. Org. Chem. 2017, 13, 1204–1211, doi:10.3762/bjoc.13.119

• Megasynthases are large multienzyme proteins that produce a plethora of important natural compounds by catalyzing the successive condensation and modification of precursor units. Within the class of megasynthases, polyketide synthases (PKS) are responsible for the production of a large spectrum of bioactive

• polyketides (PK), which have frequently found their way into therapeutic applications. Rational engineering approaches have been performed during the last 25 years that seek to employ the “assembly-line synthetic concept” of megasynthases in order to deliver new bioactive compounds. Here, we highlight PKS

• antineoplastic doxorubicin and by the antiparasitic avermectin (Figure 1a) [1]. PK are assembled from acyl-coenzyme A (acyl-CoA) units via a series of Claisen-type condensation reactions catalyzed by polyketide synthases (PKS) (Figure 1b). PKS occur as large multifunctional enzymes, termed megasynthases, which

Biosynthetic origin of butyrolactol A, an antifungal polyketide produced by a marine-derived Streptomyces

• Enjuro Harunari,
• Hisayuki Komaki and
• Yasuhiro Igarashi

Beilstein J. Org. Chem. 2017, 13, 441–450, doi:10.3762/bjoc.13.47

• hydroxymalonyl-ACP formation in the zwittermicin biosynthesis (ZmaN, ZmaD, ZmaG, and ZmaE) (Figure 6) [24] are present in the downstream of the butyrolactol PKS cluster. Genes coding for O-methyltransferase homologues responsible for O-methylation of hydroxymalonyl-ACP were not found near the cluster

• the ACP as a starter or isobutyl-CoA is used as a starter and C-methylation takes place afterwards. The signature sequence region of the acyltransferase domain of the PKS starter loading module for butyrolactol biosynthesis (FAGHS) shares some amino acid residues with the known loading module of

• coding for C-methyltransferase are not present near the butyrolactol PKS genes. Further enzymatic studies are necessary to establish the order of the starter loading/C-methylation events. Conclusion In summary, we elucidated the biosynthetic origin of butyrolactol A (1) on the basis of the feeding

Polyketide stereocontrol: a study in chemical biology

• Kira J. Weissman

Beilstein J. Org. Chem. 2017, 13, 348–371, doi:10.3762/bjoc.13.39

• (hence the name ‘modular PKS’ for this type of system), each of which carries out a single round of chain extension and chemical tailoring of the resulting intermediate. Each PKS module incorporates three functional domains necessary for chain growth (Figure 3): an acyl transferase (AT) which selects the

• positions in the polyketide chains. Building of the polyketide core is typically terminated by a thioesterase (TE) domain situated at the end of the final PKS multienzyme, which releases the product by hydrolysis or more usually macrolactonization, using an internal hydroxy nucleophile. This PKS-free

• intermediate (6-deoxyerythronolide B in the case of erythromycin biosynthesis, Figure 3) is then frequently modified by a series of so-called ‘post-PKS enzymes’ (e.g., methyl transferases, hydroxylases, and glycosyl transferases), to achieve its final bioactive form [10]. Nature has, in fact, evolved two

A non-canonical peptide synthetase adenylates 3-methyl-2-oxovaleric acid for auriculamide biosynthesis

• Daniel Braga,
• Dirk Hoffmeister and
• Markus Nett

Beilstein J. Org. Chem. 2016, 12, 2766–2770, doi:10.3762/bjoc.12.274

• microorganism as a promising source of natural products. Genes for nonribosomal peptide synthetases (NRPSs) were found to be preponderant, either solely or organised in combination with polyketide synthase (PKS) genes, representing four and five clusters, respectively. Two PKS and three putative bacteriocin

• were observed in cultures of H. aurantiacus 114-95T, providing initial evidence for the assumed secondary metabolome of this species [4][5][6]. Within the entire genus, 1 is only the second PKS/NRPS-derived molecule to be described, following the report on siphonazole (2, Figure 1) [7

• ]. Retrobiosynthetic analysis allowed the identification of a 14,130 bp-gene cluster, now referred to as aul-cluster (Figure 2), which putatively encodes two NRPSs (AulA and AulB) and one PKS (AulC) possessing domains that collectively allow and plausibly explains the assembly of 1. A gene for a type-II thioesterase

Evidence for an iterative module in chain elongation on the azalomycin polyketide synthase

• Hui Hong,
• Yuhui Sun,
• Yongjun Zhou,
• Emily Stephens,
• Markiyan Samborskyy and
• Peter F. Leadlay

Beilstein J. Org. Chem. 2016, 12, 2164–2172, doi:10.3762/bjoc.12.206

• prompted efforts to manipulate PKS domains and modules into novel combinations, as a route to obtaining novel non-natural polyketide products [6][7], and facilitated the discovery of new biosynthetic gene clusters using whole-genome sequence analysis [8][9]. A number of assembly-line PKSs do not exactly

• [13]. In trans-AT PKSs, domains are often found in unconventional order, and modules may be split between different PKS multienzyme subunits. In both types of modular PKS, domains may be present but apparently not used, or expected domains may be missing [10][12]. Perhaps the most striking deviations

• from colinearity are those where the number of modules in the PKS does not correspond to the number of extension units found in the chemical product [10][12]. Strains subjected either to random mutagenesis or to a targetted block in post-PKS steps have been found to accumulate aberrant products of