Ribosomal protein pool of Escherichia coli

Abstract

Escherichia coli B cells, which were grown in a heavy medium ( 15NH 4Cl and 2H 2O) and then transferred to a light medium ( 14NH 4Cl and 1H 2O), contained an expanded pool of ribosomal proteins. The expansion was attributable to the partial breakdown of ribosomes formed in the heavy medium due to the density-transfer and the subsequent reutilization of the released intact ribosomal proteins for the synthesis of new ribosomes. Ribosomes were prepared from E. coli cells which were subjected to the density-transfer from a heavy medium to a light medium. Newly-formed light ribosomal cores were separated from pre-existing heavy ribosomal cores by CsCl equilibrium centrifugation. It was found that newly-formed light ribosomes contained radioactive proteins labeled only during growth in the heavy medium. These proteins were shown to be ribosomal and heavy, respectively, by acrylamide-gel electrophoresis and RbCl equilibrium centrifugation. The extent of incorporation of pulse-labeled proteins into ribosomes during a chase and also the extent of incorporation of newly-formed ribosomal RNA into ribosomes after the inhibition of protein synthesis by chloramphenicol were used to estimate the size of the soluble ribosomal protein pool. Normal cells contained a pool of about 10 % of the total ribosomal proteins. After density-transfer the size of the pool is expanded to approx. 20–25 % of the total ribosomal proteins. These results provided evidence that E. coli B cells can reutilize pre-existing ribosomal proteins for the synthesis of new ribosomes.