Southern blot analyses of the argB -auxotroph mutants harboring different lengths of direct repeats for marker recycling. (a) Diagram of the AaargB disruption cassette 2. Thecandy-striped bar indicates the direct repeat sequences. The insertion of guanine (g) and adenine (a) yielded, respectively, a Nhe I restriction site and a stop codon (asterisk). (b) Left: restriction enzyme maps of the AaargB loci in the MR12 strain and its transformants with the AaargB disruption cassette 2. The DNA probes used for Southern blot analysis are indicated with a double line (5’-probe) and a solid bar (3’-probe). Right: results of Southern blot analysis using genomic DNA digested with Nhe I and Hin dIII. Lanes 1, 2, 3, and 4 are MR12, mrTA20, mrTA98, and mrTA495, respectively. (c) Left: restriction enzyme maps of the AaargB locus after the pyrG excision by intramolecular homologous recombination. Right: Southern blot analyses were performed as in (b). Lanes 1, 2, 3, and 4 are MR12 and FOA-resistant strains from mrTA20, mrTA98, and mrTA495, respectively.