Summary
Three temperature-sensitive mutant strains for RNA polymerase β or β′ subunits (carrying mutations tsx, A2R7 and R120) were used in order to investigate the dependence of the induced lac expression on stimulation by cyclic AMP after the shift to non-permissive temperature. High temperature lowered the rate of β-galactosidase synthesis. However, the low rate of synthesis could be strongly increased by cyclic AMP (30, 2.4 and 5.7-fold increases for tsX, A2R7 and R120 mutants, respectively). At the permissive temperature stimulation by cyclic AMP was less than 1.4-fold (minimal medium supplemented with glycerol). The results suggest that the maximal expression of the lac operon is saturated, that is, a hypothetical increase in RNA polymerase or cAMP-CRP concentration in the cell will not enhance the expression. The concept of saturation explains why it was possible to increase the β-galactosidase synthesis in conditions of limited promoter binding activity of RNA polymerase through increase in concentration of cyclic AMP-CRP complex in the cell (addition of cyclic AMP) to the values higher than that observed on glycerol.
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Communicated by E. Bautz
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Mandecki, W., Wild, J. Expression of the lac operon in RNA polymerase mutants of Escherichia coli K12. Molec. Gen. Genet. 173, 339–343 (1979). https://doi.org/10.1007/BF00268645
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DOI: https://doi.org/10.1007/BF00268645