Biosynthesis of the β and β′ subunits of RNA polymerase in Escherichia coli

Abstract

The amounts of the β and β′ subunits of the DNA-dependent RNA polymerase relative to the amount of total protein synthesized have been determined under a number of growth conditions in two strains of Escherichia coli. The results of these measurements have been expressed as the relative rate of synthesis of core RNA polymerase, α p, assuming the four constituent subunits (2α, 1β and 1β′) to be synthesized in equivalent amounts. This quantity, α p, was found not to vary greatly with the growth rate μ. For glucose-grown cells of E. coli B/r (μ = 1.5 doublings/h) α p = 1.4%, corresponding to about 7000 molecules of core RNA polymerase per cell. For slowgrowing cells the value obtained for α p is lower and for fast-growing cells somewhat 3 higher. The comparison of these values with the number of RNA polymerase molecules estimated to be actively engaged in RNA synthesis indicates that both slow- and fast-growing cells contain a surplus of RNA polymerase, if the catalytic unit is assumed to be the monomer of core RNA polymerase. In addition to the measurements of cells during balanced growth at various rates, α p has been determined during the transition from one growth rate to another and during synchronous growth. During a shift-up the rate of synthesis of polymerase follows closely the rate of total protein synthesis, α p being nearly constant for a period of twenty minutes after the shift. In a synchronously dividing culture of E. coli B/r, α p was seen to be fairly constant during two cycles of synchronous division. It appears that α p is rather insensitive to the effect of gene doubling during the cell cycle.