Expression of Pseudomonas fluorescens D-galactose dehydrogenase in E. coli

To investigate the heterologous expression of Pseudomonas genes in Escherichia coli we have cloned P. fluorescens DNA in an E. coli [cosmid] system. A colony bank representing the whole P. fluorescens chromosome was screened immunologically using a modification of the method described by Broome and Gilbert (1978). Radioactive labelling of the antibodies was replaced by conjugation with horseradish peroxidase. Among 523 E. coli colonies one was D-galactose dehydrogenase-positive. The expression of this enzyme in primary clones was lower than in the uninduced Pseudomonas. Subcloning of the D-galactose dehydrogenase gene, in vitro mutagenesis of the DNA, and coupling to a strong E. coli promoter yielded an E. coli strain that produces 90 times more of the enzyme than the induced P. fluorescens.