The Concentration of Polypeptide Chain Release Factors 1 and 2 at Different Growth Rates of Escherichia coli

Abstract

The number of molecules of release factor-1 (RF-1) and release factor-2 (RF-2) per Escherichia coli cell grown at various rates was determined using quantitative Western blotting of total solubilized cell protein. The number of RF-1 molecules per cell increased from 1200 to 4900, and of RF-2 from 5900 to 24,900 as growth rates increased from 0·3 to 2·4 doublings per hour. The cellular concentration of the release factors, and therefore efficient termination of protein synthesis is maintained by the increased expression of both RFs as growth rate increases. The expression of both release factors RF-1 and RF-2 is co-ordinated with that of the rest of the translational apparatus, although the increases are less for RF than that for the ribosomes under the same conditions. A significant proportion of the RF pool was found associated with the ribosome fraction. The percentage of ribosomes containing an RF molecule increased from 21 to 33% as the translational rate increased over the growth rate range. Since the cellular concentration of the release factors and their specific activity does not vary significantly with growth rate, this can not provide for an increase in the rate at any of the steps of termination. The postulated strong stop signals, UAAU and UAAG, in genes that are highly expressed at fast growth rates, may result in an increase in the termination rate as a consequence of increased efficiency of decoding by Rfs.