Stability of Escherichia coli strains harboring recombinant plasmids for L-threonine production.

Escherichia coli recombinant strains bearing the thr operon have been previously selected for threonine production and phenotypically classified according to antibiotic resistance properties (Nudel et al. 1987). Further analysis of those strains permitted the isolation and restriction mapping of two different plasmids of 13 kb and 18.6 kb. The smaller one, which expressed tetracycline resistance gave better results on threonine accumulation but it was rather unstable when grown without antibiotic pressure. Therefore, other hosts were transformed with those plasmids to improve stability. A threonine-auxotrophic strain was a better host for plasmid maintenance and expression of thr operon. Host influence in plasmid-mediated threonine production was studied in terms of specific yields (the ratios of threonine accumulated to biomass values) and of plasmid maintenance (percent of AprTcr clones after cultivation in non selective media). We also determined that semisynthetic media of defined composition were better than rich media for threonine expression, due to feed-back controls exerted by undesired catabolites accumulated in complex media.