**5.3 Identification and characterization of Type II polyketides**

Type II polyketides are a group of small molecules with aromatic rings and contain alternating carbonyl and methylene groups (-CO-CH2-).Many of the Type II polyketides (e.g. tetracycline and doxorubicin) are well documented for antimicrobial and ant cancerous activities [90]. Gene clusters involved in synthesis of these small molecules are rather divergent and exhibit low levels of DNA sequence homology, yet each of them contain at least a 'polyketide synthetase', encoded by three highly conserved genes, i.e. 2 genes for ketosynthases (KSs) and one gene for a acyl carrier protein. These 3 genes are referred as 'minimal PKS synthesis gene cluster'. Studies carried out with metagenomes in general and extremophilic metagenome in particular have shown a rich diversity of novel 'minimal PKS synthesis gene cluster' [102]. In subsequent studies, gene clusters with minimal PKS synthesis genes were identified in soil metagenomes [103]. The transformation and heterologous expression in different strains belonging to genus *Streptomyces* and lead to synthesis and identification of several new polyketide metabolites with previously unknown and rare carbon skeletons [93].
