Biosynthesis of actinorhodin

Kendrew, S.G., Hopwood, D.A. and Marsh, E.N.G. (1997) Identification of a monooxygenase from Streptomyces coelicolor A3(2) involved in biosynthesis of actinorhodin purification and characterization of the recombinant enzyme. Journal of Bacteriology, 179, 4305—4310. 

The aromatization often takes place after linear precursors have been modified as in the biosynthesis of actinorhodin, where condensation followed by ketone reduction takes place before aromatization (Scheme 7.7) [31]. 

As mentioned above, iterative PKSs have been studied in detail, focusing especially on the biosynthesis of actinorhodin and tetracenomycin. Later, further genes and gene clusters have become available. In this section we describe how knowledge of the function of these enzymatic systems can be used to generate new molecules. The most important results are summarized in Figure 12.1 la and b. 

Fig. 1.13 Biosynthesis of actinorhodin (16), an example of ein iterative type II PKS. During the biosynthesis eight malonyl-CoA units are sequentially condensed to produce half of the dimeric actinorhodin molecule...

S. Okamoto, T. Taguchi, K. Ochi, K. Ichinose, Biosynthesis of actinorhodin and related antibiotics discovery of alternative routes for quinone formation encoded in the act gene cluster. Chem. Biol. 16, 226-236 (2009)... 

SCHEME 2.10 Control of the regiochemistry in ring formation in the biosynthesis of actinorhodin (43). 

Kim E-S, Hong H-J, Choi C-Y, Cohen SN, Modulation of actinorhodin biosynthesis in Streptomyces lividans by glucose repression of afsR2 gene transcription,... 

Figure 3 Aromatic PKS gene clusters and actinorhodin biosynthesis. Partial gene clusters for the polyketides actinorhodin (act), tetracenomycin (tcm), and doxorubicin (dps) are shown. Reconstitution of combinations of act genes in a PKS clean host have led to a proposed pathway by which the early stages of actinorhodin biosynthesis occur.

PL Bartel, CB Zhu, JS Lampel, DC Dosch, NC Conners, WR Strohl, JM Beale, HG Floss. Biosynthesis of anthraquinones by interspecies cloning of actinorhodin biosynthesis genes in Streptomyces. clarification of actinorhodin gene functions. J Bacteriol 172 4816-4826, 1990. 

In similar experiments, cell-free preparations of a Streptomyces coelicolor strain containing the minimal PKS involved in actinorhodin biosynthesis were shown to produce compounds called SEK4 and SEK4b, molecules generated as shunt products of the normal pathway (Fig. 7). When the strain also included genes for the KR, ARO, and CYC activities, a more elaborate structure, 3,8-dihydroxy-l-methylanthraquinone-2-carboxylic acid (DMAC), was obtained (Fig. 7) [8], Biosynthesis of SEK4 and SEK4b has also been accomplished in... 

Figure 7 Cell-free biosynthesis of SEK4, SEK4b, and DMAC. A strain of Streptomyces coelicolor in which the actinorhodin-minimal PKS genes had been overexpressed, produced both SEK4 and SEK4b. Expression of the KR, ARO, and CYC genes in addition to the minimal PKS resulted in the biosynthesis of DMAC. Synthesis of SEK4 and SEK4b has also been achieved using purified proteins in vitro.

Type II PKSs, on the other hand, carry out polyketide synthesis in an iterative manner, and are responsible for the biosynthesis of aromatic polyketides, as exem-phfied by the actinorhodin PKS (Pig. 5.2 C) and R1128 PKS (Pig. 5.2D). Each active site occurs as a distinct monofunctional protein, and the entire PKS complex is used for aU the synthesis steps. It should be noted that that some PKSs have features of both type 1 and type 11 categories. Por a review of this topic, see Ref. [39]. 

Fig. 5.2 Organization of polyketide synthases (PKSs). (A) The type I erythromycin PKS (DEBS) which catalyzes the biosynthesis of 6-dEB. The PKS consists of three polypeptides, DEBSl, DEBS2, and DEBS3, each possessing two modules. (B) The type I epothilone PKS consists of six polypeptides EpoA, EpoB [a non-ribosomal peptide synthase (NRPS)], EpoC, EpoD (possessing four modules), EpoE (possessing two modules), and EpoF. (C) The type II actinorhodin PKS con-...

The most well-established system for heterologous expression involves the hosts S. coelicolor or its close relative S. lividans, and a bifimctional actino-myces- . coli vector with control elements for PKS gene expression that have been derived from the actinorhodin gene cluster [59]. This host-vector system has successfuUy been used to reconstitute functionally the polyketide pathways associated with biosynthesis of frenolicin [60], tetracenomycin [59], oxytetra-cycline [61], erythromycin [62], picromycin/methymycin [63], oleandomycin... 

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