Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTION1.7.2.1 Components of fermentation mediaTwo forms of liquid media can be used in the production of secondary metabolites,these include chemically defined (synthetic) or undefined (natural, complex).Inexpensive complex media are usually employed in commercial fermentations whichgive higher fermentation yields at a lower cost (Dahod, 2000). However, complexmedia contain ill-defined components which lead to lot-to-lot variability and mayalter expression profiles of metabolites in actinobacteria (Zhang and Greasham,1999).1.7.2.2 Macronutrients1.7.2.2.1 Carbon SourceCarbon sources tend to display species-specific variation in actinobacteria for cellgrowth and production of secondary metabolites (Dekleva et al., 1985; Demain, 1989;Platas et al., 1999). The repressive negative effects of certain carbon sources onsecondary metabolite production is due to carbon catabolite repression. It occurs inmany actinobacterial species [Tables 11 and 12] (Stülke & Hillen, 1999). Rapidlyassimilated carbon sources such as glucose are good for growth, but it represses theformation of enzymes involved in the biosynthesis of secondary metabolites (Table11). Polyalcohols (e.g. glycerol), polysaccharides (e.g. starch), oligosachharides (e.g.lactose) and oils (e.g. soybean, methyloleate) are often used as preffered nonrepressingcarbon sources as the carbon source is released more slowly by hydrolysis(Trilli, 1990; Demain & Fang, 1995).Table 11. Inhibition of secondary metabolism by nutrientsSecondaryProducerNutrient ReferenceMetaboliteInhibitorStreptomycin Streptomyces griseus Glucose Demain, 1989Kanamycin Streptomyces Mannose, Lactose Demain, 1989kanamyceticusThienamycin Streptomyces cattleya Glutamine Paress andStreicher, 1985Candicidin Streptomyces griseus Phosphate Martin, 1989Glycerol is well known for its stabilising effect on secondary metabolites,biosynthetic enzymes and causing increased SM yields in actinobacteria (Arroyo etal., 2000; Chen et al., 2002). Alternative less expensive carbon sources which have_____________________________________________________________________38