Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTION(KR), aromatase (ARO) and cyclase (CYC) introduce chemical structural diversity tothe polyketide structure (Alvarez et al., 1996). The gene clusters also containtranscriptional activator or regulatory genes, as well as self-resistance genes (Figure9).A)B)SEK4bFigure 8. Aromatic polyketide biosynthesis of the gene products from theactinorhodin PKS cluster. (A) Presence of the minimal PKS is sufficient for theproduction of two 16-carbon polyketides, SEK4 and SEK4b.(B) The nascentoctaketide chain undergoes ketoreduction (catalyzed by the KR), aromataization ofthe first ring (catalyzed by the didomain ARO/CYC) and a second cyclization(catalyzed by the second ring cyclase (CYC2). (adapted from Zawada & Khosla,1997).Aromatic polyketide biosynthesis begins with a primer unit loading on to thecondensing β-keto acyl synthase (KS; figure 8B). An extender unit is then transferredto the pantetheinyl arm of the acyl carrier protein (ACP; figure 8B). The KS catalyzesthe condensation between ACP-bound malonate and the starter unit. Extender unitsare added sequentially until the polyketide chain reaches a desired chain lengthdetermined by the chain length factor. The ketoreductase can then catalyze reductionof the C-9 carbonyl (AT; figure 8B). Aromatase and cyclase (ARO/CYC; figure 8B)then catalyzes the aromatisation of the first ring in reduced polyketides. The secondring undergoes a C-5/C-14 cyclization by the actions of an cyclase in reducedpolyketides. The primary in vivo product of the complete actinorhodin PKS is 3,8-dihydroxy-1-methylanthraquinone-2-carboxylic acid (DMAC). In the absence of_____________________________________________________________________17