Use of gene fusions to study expression of cysB, the regulatory gene of the cysteine regulon.

Strains of escherichia coli were constructed in which the lacZ gene is fused to cysB, the positive regulator gene of the cysteine regulon. The fusion strains were used to study the regulation of the cysB gene by assaying the fused lacZ gene product. The introduction of a cysB allele, either on a plasmid or on an episome to the fusion strains, resulted in the decrease of beta-galactosidase activity. This implies that the cysB gene expression is autoregulated by its own product. The direction of cysB gene transcription was determined to be clockwise.