Summary: The causes of the instability of a multicopy plasmid, pCT70, which directs the expression of calf prochymosin in were investigated. Plasmid pAT153 and its derivative, pCT54, were stable for more than 90 generations in continuous culture with glucose limitation. The multicopy plasmid pCT66, which expressed very low levels of prochymosin due to poor translational efficiency, and low copy number plasmids which efficiently expressed the prochymosin gene, were also stable. These results indicated that high level translation of the recombinant gene was the cause of the instability of pCT70. The maximum specific growth rate of (pCT70) was reduced by 30% compared with (pCT66).

To fulfil the requirements of a production system, a dual origin plasmid with controllable copy number was developed. Both this plasmid (pMG165) and a derivative which contained the prochymosin gene (pMG168) were stable when maintained at low copy number. When the copy number of plasmid pMG168 was increased by putting replication under the control of the λP promoter and the temperature sensitive repressor, expression of prochymosin was achieved. This strategy enables large-scale production of prochymosin without the need for antibiotic selection or other methods of preventing plasmid loss.


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