Highly efficient production of enzymes of an extreme thermophile, Thermus thermophilus: A practical method to overexpress GC-rich genes in Escherichia coli

The GC-rich leuB gene (coding for 3-isopropylmalate dehydrogenase) of Thermus thermophilus is scarcely expressed in Escherichia coli. unless a leader open reading frame (ORF) is provided. We conducted experiments on nonexpressible plasmids and obtained a modified plasmid showing greatly enhanced expression: the degree of expression from the plasmid was higher than that from any other plasmid so far constructed. Sequence analysis of the plasmid showed that a 258-bp leader ORF overlapped with the initiation codon of leuB was newly formed as a consequence of the insertion of a 0.5-kb BamHI fragment derived from the E. coli chromosome. The degree of expression from the plasmid was further improved by shortening the leader ORF to 36 bp without changing the overlapping portion, and the flanking sequence between the promoter and the leader ORF was removed. The expression in E. coli of the pfk1 gene (coding for phosphofructokinase) of T. thermophilus was improved by the construction of a structure similar to that which enhanced the expression of the leuB gene. Based on the results, a practical method for the overexpression of GC-rich genes in E. coli is proposed.