Plasmid maintenance and threonine production by recombinantEschcrichia coli strains

Summary E. coli NRRL 12100-a recombinant strain obtained by genetic manipulationwas used forL-threonine production. When cultured in a rich medium without antibiotics three types of colonies were isolated (Ap^sTc^s, Ap^rTc^s, and Ap^rTc^r). The Ap^rTc^r clones were best threonine producers (9 to 12 g/1) and the plasmid was maintained, in 65 to 93% of the host cells. When inocula were grown under selective pressure we obtained about 10 g/1 of threonine and a plasmid maintenance of 83%. When growth of inocula was done without antibiotic, threonine yields dropped to 5 g/1 and 64% of the cells have lost the plasmid. Batch culture experiments were performed with 3, 4 and 6% of glucose, added at the initial stage or in a discontinuous feed. Threonine yields and plasmid stability were not affected. The elimination of the maximum level of threonine produced (from 13.8 to 6.7 g/1) and on the plasmid maintenance (from 94 to 4% of the cells) while growth of the strain was not affected.