Segregational stability of the recombinant plasmid pKA18 bearing the pga gene encoding penicillin G acylase (PGA) and the production stability for PGA was studied in Escherichia coli strain W and its descendants. The hosts differ in the number of indigenous plasmids (pRK1-3) and the specific activity of PGA. The production of PGA by the recombinant strain RE3(pKA18) is the highest among the hosts and the increase of activity is proportional to the increase of copy number (CN) of the plasmid pKA18. The plasmid is segregationally stable in spite of the presence of the indigenous plasmid pRK2 that belong to the same CoIE1 plasmid family. Both plasmids (total sum of CN equals 271) are maintained without selection pressure during 102 generations. Subculture-based selection experiments taking advantage of a very high segregational and production stability of plasmid pKA18 in the host RE3 were used to study the optimization of the host-plasmid interaction. The clones with increased synthesis of the enzyme average value of 833 U/g DM were isolated: the expression of PGA per plasmid molecule increased in these strains from 3 to 4 U. Once this expression is reached, the population of segregationally stable, high-producing strains becomes heterogeneous: segregationally stable, low-producing clones and segregationally unstable, high-producing clones appear in population. (C) 2004 Elsevier Inc. All rights reserved.