Amino Acid Precursor Supply in the Biosynthesis of the RNA Polymerase Inhibitor Streptolydigin by Streptomyces lydicus

Biosynthesis of the hybrid polyketide-nonribosomal peptide antibiotic streptolydigin, 3-methylaspartate, is utilized as precursor of the tetramic acid moiety. The three genes from the Streptomyces lydicus streptolydigin gene cluster slgE1-slgE2-slgE3 are involved in 3-methylaspartate supply. SlgE3, a ferredoxin-dependent glutamate synthase, is responsible for the biosynthesis of glutamate from glutamine and 2-oxoglutarate. In addition to slgE3, housekeeping NADPH-and ferredoxin-dependent glutamate synthase genes have been identified in S. lydicus. The expression of slgE3 is increased up to 9-fold at the onset of streptolydigin biosynthesis and later decreases to similar to 2-fold over the basal level. In contrast, the expression of housekeeping glutamate synthases decreases when streptolydigin begins to be synthesized. SlgE1 and SlgE2 are the two subunits of a glutamate mutase that would convert glutamate into 3-methylaspartate. Deletion of slgE1-slgE2 led to the production of two compounds containing a lateral side chain derived from glutamate instead of 3-methylaspartate. Expression of this glutamate mutase also reaches a peak increase of up to 5.5-fold coinciding with the onset of antibiotic production. Overexpression of either slgE3 or slgE1-slgE2 in S. lydicus led to an increase in the yield of streptolydigin.