Abstract
The properties of ribonucleoprotein particles which have a rapid turnover and are considered as ribosomal precursors have been studied in Escherichia coli under conditions of simultaneous nucleic acid and protein synthesis resembling exponential growth.
In 10−2 m-magnesium ion concentration, high specific activity of pulse-labeled protein is found in particles sedimenting around 30 and 50 s, which are highly sensitive towards ribonuclease, in contrast to ribosomes and their 30 and 50 s subparticles. They take up newly synthesized protein with a very short time lag (ten seconds) comparable to the time required for the synthesis of a protein. This indicates that newly synthesized protein is transferred directly to the precursors, and that these ribonucleoproteins do not contain the nascent peptides and therefore are not themselves sites of protein synthesis.
A large pool of free ribosomal proteins is excluded since most of the precursor proteins are shown to be chased into the ribosomes within a few minutes.
The evidence suggests direct transfer for the major part of ribosomal proteins from producing ribosomes to ribosomal precursors.
