Effects of magnesium ions on beta-galactosidase production and on transcription in escherichia coli

Abstract

The data which have been presented are consistent with a previous report that the concentration of magnesium ions in the cell sap of E. coli is the same as the concentration in the growth medium over a range of 1 μm to 10 μm.

The above data are not consistent with previous reports that the concentration of unbound magnesium ions in the cell-sap of E. coli remains constant as the external magnesium ion concentration is varied. The data are also inconsistent with the theory that E. coli cells carry out active transport which extrudes magnesium ions from the cell as the external concentration is raised above 5 mM. The active transport theory predicts that increases in extracellular magnesium ion content will not be reflected internally and, therefore, should not alter the rates of any internal processes. The results of the foregoing experiments do not eliminate the possibility that E. coli cells possess an energy dependent system which accumulates magnesium ions whenthe level in the growth medium falls below 0.4 mM.

Additionally, in the presence of rifampin, the amounts of RNA in both the stable and unstable fractions of total cellular RNA that undergo elongation are reduced equally by increasing the extracellular magnesium ion level. Finally, the rate of elongation of total cellular RNA was compared to the rate of total RNA synthesis (comprised of both initiation of new RNA and elongation) as the external magnesium ion concentration is raised. There is also an indication that the initiation phase of total cellular RNA synthesis is inhibited.

The observed inhibitory effects of magnesium ions on growth and protein synthesis in these two strains of E. coli may reflect the fact that increased extra cellular magnesium ions cause the cell sap (S-1OO fraction) magnesium ion concentration to rise which may then alter internal physiological reactions. The effect of increasing magnesium ion concentration above 5 rmM on the induced synthesis of beta-galactosidase was investigated in order to localize which, if any, specific intracellular reactions are inhibited by increased extracellular magnesium ions. Increasing the concentration of magnesium ions in the growth medium beyond 5 mM results in a progressive decrease in the rate of induced beta-galactosidase formation by E. coli B cells. It appears that the process of beta-galactosidase production that is sensitive to increasing external magnesium ions is transcription of the z gene. The initiation phase of transcription was found to bel,moresensitive to increased extracellular magnesium ion concentrations than the elongation phase. The data obtained by further experimentation revealed that the overall rate of synthesis of cold trichloroacetic acid insoluble nucleic acids by E coli AS 19 cells is progressively reduced by increasing the magnesium ion concentration in the growth medium from 5 to 20 mM.

This investigation has revealed that increasing the magnesium ion content of the growth medium from 5 to 20 millimolar results in a progressive reduction in the growth rate of an E. coli B culture both in nutrient broth and a mineral salts medium. An inhibitory effect has also been seen on the growth rate of E. coli AS 19 cells grown in an inorganic salts medium. Further experiments revealed that the overall rate of protein production in both E. coli B cells and E. coli AS 19 cells was inhibited by increasing the extracellular magnesium ion concentration above 5 rmM. This was determined from measurements of both the total protein content and the rate of incorporation of ¹⁴C-leucine.

The effect of increasing the extracellular magnesium ion content over the range from 5 rmM to 20 mM has been observed on the growth rate, overall rate of synthesis of total cellular proteins, induced synthesis of beta-galactosidase, and on the production of total cellular RNA in E. coli cells. Two strains of E. coli cells (E. coli B and E. coli AS 19) have been employed. E. coli AS 19 is a derivative of E. coli B that is sensitive to the antibiotic rifampin without requiring the additional presence of a chelating agent.