Abstract

A hybrid ColE1 plasmid [pYe(trp5)1], containing a yeast DNA segment that complements auxotrophic point mutations and deletions in the Escherichia coli tryptophan synthetase gene (trpAB), has been isolated. Expression of the yeast tryptophan synthetase activity from the cloned yeast gene (trp5) is relatively inefficient in E. coli, as measured by growth rates of trpAB/pYe(trp5)1 strains on minimal media lacking tryptophan and by enzyme assays. Faster growing variants occur spontaneously at a frequency of one in 10(4)--10(5) cells plated and produce higher levels of the yeast enzyme. Plasmid DNA [pYe(trp5)2] from one of these variants was shown to contain a DNA insertion (1.3 kilobase pairs) in the cloned yeast DNA segment in relatively close proximity to the trp5 gene. This DNA insert was identified as a bacterial IS2 element, which carries a promoter for RNA transcription when inserted in the proper orientation. The spontaneous integration of a bacterial DNA insertion element into cloned eukaryotic DNA can result in more efficient expression of the foreign gene.