CII-dependent activation of the pRE promoter of coliphage lambda fused to the Escherichia coli galK gene.

Using a cloning vector designed for the study of prokaryotic promoters by fusion to the Escherichia coli galactokinase gene (galK), we have constructed a plasmid in which the lambda pRE promoter controls galactokinase expression. A galK- host containing this plasmid has a Gal- phenotype since transcription from pRE requires activation by the lambda CII protein. When CII protein is provided by a prophage, galactokinase is synthesized at a rate dependent on the concentration of CII protein. A second plasmid was constructed in which the pRE promoter from phage 21 controls galactokinase expression. Transcription of the galK gene in this plasmid requires the phage 21 CII protein. Using this system, we demonstrate that the lambda and 21 pRE promoters are highly selective for their corresponding CII proteins. However, a cross-reaction between 21 pRE and the lambda CII protein was observed. In addition, we transferred the pRE-galK fusion unit from the plasmid to a phage, and then to the host chromosome in single copy. Galactokinase expression in this single copy pRE-galK system is also dependent on CII protein, which may be provided from a multicopy plasmid. The high concentration of CII protein provided by the plasmid results in maximal expression of the pRE-galK transcription unit. In this second system low levels of CII activity from CII- mutants are amplified and can be readily detected.