(A) Scheme representing the replacement in the chromosome of the wild-type slgE3 gene by a version mutated through the insertion of an apramycin resistance cassette. (B) UPLC analysis of mutant SLME3. (C) UPLC analysis of mutant SLME3 complemented by pEM4TslgE3. (D) Scheme representing the replacement of the wild-type slgE1-E2 genes by one mutated through the insertion of an apramycin resistance cassette. (E) UPLC analysis of mutant SLME1E2 and MS analysis of compounds 2 and 3. (F) UPLC analysis of mutant SLME1E2 complemented by pEM4TslgE1E2. aac(3)IV, apramycin resistance gene; hyg, hygromycin resistance gene; bla, β-lactamase gene; 1, streptolydigin; 2, streptolydigin B; 3, streptolydigin C; AU, arbitrary units.