Polarity suppressors increase expression of the wild-type tryptophan operon of Escherichia coli

Three new polarity suppressors, selected to relieve the polar effect of nonsense mutations in the tryptophan (trp) and lactose (lac) operons of Escherichia coli, increase expression distal to nonsense mutations in both operons to a greater extent than suA. These suppressors relieve the polarity created by amber, ochre and frameshift mutations with equal efficiency.Two of the three polarity suppressors elevate enzyme synthesis in the wildtype trp operon two- and fivefold, respectively. The increase in enzyme levels is in each case correlated with increased levels and rates of synthesis of structural gene trp messenger RNA. Since expression of all genes is elevated, these findings suggest the existence of a site early in the wild-type trp operon that affects the extent of operon expression. We located the site affected by these two polarity suppressors between the operator and the first structural gene, trpE. Although the third polarity suppressor also relieves mutational polarity efficiently, it has no detectable effect on expression of the wild-type trp operon.     

Stationary phase expression of the arginine biosynthetic operon argCBH in Escherichia coli

Background  

Arginine biosynthesis in Escherichia coli is elevated in response to nutrient limitation, stress or arginine restriction. Though control of the pathway in response to arginine limitation is largely modulated by the ArgR repressor, other factors may be involved in increased stationary phase and stress expression.     

DNA-dependent in vitro synthesis of enzymes of the galactose operon of Escherichia coli

Summary Two active enzymes of the galactose operon of Escherichia coli, uridyl transferase and galactokinase have been synthesized with high yields in a DNA dependent system for protein synthesis. The unspecific blank values amount to less than two percent of the rate obtained under optimal conditions and permit the accurate determination of even a small fraction of the maximum synthesis rate. Therefore this system provides a sensitive assay for the biological activity of DNA that contains the intact galactose operon of Escherichia coli.The synthesis of these galactose enzymes is to a high extent dependent on the presence of cyclic adenosine-3:5-monophosphate.D-fucose, known as an inducer of the galactose operon in vivo, stimulates the synthesis of galactokinase, indicating that the repressor of the galactose operon in active under these conditions. This stimulation is not observed, if the bacterial extract is prepared from a strain defective for the galactose repressor or if the DNA carries an operator constitutive mutation in the galactose operon. Therefore the stimulation by D-fucose is true derepression.