Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTIONActinobacteria contain two distinct types of polyketide synthases, either type I PKS(multifunctional), which contain a single large protein encoded by a single gene thatcontains the entire set of catalytic functions (Figure 6), or a type II PKS(monofunctional), which contain several separate proteins that perform specificcatalytic functions [see figure 8A] (Hutchinson & Fuji, 1995).1.3.1.3 Biosynthetic Gene Clusters of Type I Polyketide SynthasesA characteristic feature of type I PKS genes is that they contain large open readingframes encoding giant (>3000 amino acids) multifunctional proteins (Cortes et al.,1990). Within these proteins are specific catalytic centers known as “modules” whichare present as single copies and contain specific active sites which determine thebiochemical order of substrate selection for chain-elongation steps and the degree ofβ-ketoacyl reduction of each extension step (Figure 6). The regions between themodules have been implicated to serve as structural supports maintaining correctformation for the catalytic process (Katz, 1997; Staunton & Wilkinson, 1999).Figure 6. General modular organisation of the type I PKS. Abbreviations (LD)loading domain; (M) modules; (ORFs) open reading frames; (AT) acyl transferase;(ACP) acyl-carrier-protein; (KS) ketosynthase; (DH) dehydratase; (ER) enoylreductase; (KR) ketoreductase (adapted from Lal et al., 2000).The AT domain selects the type of extender unit (acetate or priopionate) to beincorporated into the growing chain; the KS domain catalyses the condensationreaction and the ACP domain tethers the polyketide chain to the PKS betweencondesation cycles and accepts the extender unit from the AT domains in readinessfor the next condensation event. The keto ester produced by these core domains isprocessed by optional modifying reductive domains (ER, KR and DH). The extent towhich β-reduction occurs is determined by the number of reductive domains present.The loading module which precedes the first module is responsible for activating thestarter unit and making it available to the PKS (Lal et al., 2000; Katz et al., 1997)._____________________________________________________________________14