Overproduction of proteins encoded by lambda replicon-integrated plasmid: effect of partial inactivation of cI...

Composite plasmids containing the 7.2-kb fragment of bacteriophage lambda cI857cro27, essential for lambda DNA replication and its control, have been constructed. The plasmids contained, in addition, pBR322, a part of Co1E1, and the Escherichia coli pss gene, coding for phosphatidylserine synthase. When the plasmid-harboring cells were induced at different temperatures, the phosphatidylserine synthase production was maximum at 37 degrees C, and the specific activities at 37 degrees C continuously increased until the culture reached saturation. The results suggest that, when the cro gene is defective, only a transient temperature for inactivation of cI repressor provides both the increase in copy number of the plasmids and cellular activities that allow overproduction of the plasmid-encoded protein.