Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTIONpathways; (c) phosphate limits synthesis of the inducer of the SM pathway (Martín,1989); (d) phosphate inhibits the formation of SM precursors (Martín, 1977); (e)phosphate inhibits or represses phosphatases necessary for SM biosynthesis; (f)phosphate suppresses SM production by depriving the cell of an essential metal(Martín et al., 1989). In liquid media SM biosynthesis is repressed or inhibited byPO 3- 4 concentrations above 1 mM whereas in solid media higher concentrations of 10to 25 mM are required (Martín, 1989). The synthesis of biosynthetic enzymes areaffected by PO 3- 4 at the transcriptional level [Table 13] (Reeve & Baumberg 1998). Insome instances, excess of glucose and phosphate act synergistically causingrepression of SM biosynthesis (Lounès et al., 1996).Table 13. Phosphate-regulated enzymes involved in secondary metabolitebiosynthesis (Adapted from Martín et al., 1994).SecondaryMetaboliteProducingOrganismTarget Enzyme Mechanism ofregulation*Candicidin Streptomyces griseus p-Aminobenzoate synthase RCephamycin Streptomyces clavuligerus Deacetoxycephalosporin C^ IIsopenicillin N synthase^INeomycin Streptomyces fradiae Neomycin phosphatephosphotransferaseR* R, repression; I, inhibition^ Enzymes involved in cephamycin biosynthesis are less sensitive to phosphate control (concentrationsof more than 25 mM phosphate are required to observe phosphate control) than are other secondarymetabolite biosynthetic enzymes (usually sensitive to less than 5 mM phosphate).1.7.2.2.4 Sulphur, Potassium, Magnesium SourcesSulphur is a component of proteins and prosthetic groups (-SH) of some biosyntheticenzymes and coenzyme A. Actinobacteria producing secondary metabolitescontaining sulphur atoms such as cephamycin and cyclooctasulfur have preferencestowards the source of sulphur they utilise. Amino acids such as L-cysteine and L-cystine have been effectively used in enhancing yields of cyclooctasulphur in S.albulus though inorganic sulphur salts such as sodium sulfite and sodium thiosuphatesuppressed production (Hayashi et al., 1985). Conversly, inorganic salts were goodsources of sulphur for cephamycin biosynthesis in S. clavuligerus and S.lactamdurans, whereas amino acids sources were not effective (Romero et al., 1984).A useful inorganic sulphur source for SM fermentations is ammonium sulphate[(NH 4 ) 2 SO 4 ] which can be used concomitantly as the nitrogen source (Dunn, 1985).Inorganic potassium K + cation is a cofactor of some SM biosynthetic enzymes and is_____________________________________________________________________40