DNA plasmids with improved expression

A method for heat inducible production of a target protein or mRNA gene from the Escherichia coli genome using a temperature sensitive lambda repressor repressed PL promoter, comprising:

• a. Cloning the said target protein or mRNA gene into an expression vector to create a PL-target protein or PL-mRNA gene expression cassette in which said target protein or mRNA gene is expressed under the control of the PL promoter and said PL promoter further comprises a OL1 mutation selected from the group comprising PL promoter OL1-G (SEQ ID NO:
• 11), and PL promoter OL1-G to T (SEQ ID NO:
• 12);
• b. Integrating said PL-target protein or PL-mRNA gene expression cassette into a host strain genome to create a PL-target protein or PL-mRNA host strain in which expression from the said PL-target protein or PL-mRNA gene expression cassette is repressed by a temperature sensitive cl857 lambda repressor;
• c. propagating said PL-target protein or PL-mRNA host strain at 25-32° C. to maintain said target protein or mRNA at a basal copy number;
• and
• d. inducing said PL-target protein or PL-mRNA host strain at 37-42° C. to increase expression of said target protein or mRNA.