Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTION1.7.2.9 AutoregulatorsLow molecular weight hormone-like signal substances, known as autoregulators cancontrol the production of secondary metabolites (Table 16). This effect is exerted byeither the repression or inhibition of secondary metabolite synthases and enzymessupplying biogenetic precursors (Shapiro, 1989). These autoregulators in manyinstances are able to restore simultaneously secondary metabolite production inblocked mutants of producing actinobacteria (Trilli, 1990).Table 16. Autoregulators effecting secondary metabolite production in actinobacteria.Autoregulator Autoregulator Autoregulator Effects on ReferencesTypegroupsecondarymetabolismAntibiotic S-component of peptidolactone antibiogenisis Biot, 1984VirginiamycinNon-antibiotic A-factor γ-butyrolactone antibiogenisis Horinouchi, 1985ProteinaceousFactorsFactor-C - alteringbiosyntheticBiro et al., 1980Nucleotideanaloguesantibioticspathwaystunicamycin pyrimidinoid antibiogenisis De Carvalho andMolinari, 19831.7.2.10 Miscellaneous Factors Influencing Secondary MetaboliteProductionOsmotic stresses such as high sugar or salt conditions have been shown to control SMproduction (Elibol & Mavituna, 1998). Incompatibilities of medium components canlead to precipitation of macro- and micronutrients particularly Mg 2+ and PO 4 3- . Thisprecipitation of media components are unavailable to the cell and so deficiencies canoccur. Controlling the concentrations of trace elements is achieved by chelation,which act as metal ion buffers maintaining a constant supply at physiologicalconcentrations to the growing cells. The amount of chelating agent used in afermentation is usually empirically determined, the types of chelating agents used inmedia affects the rate of SM production in chemically defined media. EDTA (as adisodium salt) is a popular choice and in complex media citrate or nitrilotriacetic acidis commonly used (Melzoch et al., 1997).Inoculum types (spores or vegetative cells), concentrations and morphologysignificantly effects SM production and varies for individual actinobacterial cultures(Elibol et al., 1995; Neves et al., 2001; Jonsbu et al., 2002). In the case of the polyene_____________________________________________________________________46