The kinetics of the synthesis of ribosomal RNA in E. coli.

The kinetics of the synthesis of ribosomal RNA in E. coli has been studied using C(14)-uracil as tracer. Two fractions of RNA having sedimentation constants between 4 and 8S have kinetic behavior consistent with roles of precursors. The first consists of a very small proportion of the RNA found in the 100,000 g supernatant after ribosomes have been removed. It has been separated from the soluble RNA present in much larger quantities by chromatography on DEAE-cellulose columns. The size and magnitude of flow through this fraction are consistent with it being precursor to a large part of the ribosomal RNA.A fraction of ribosomal RNA of similar size is also found in the ribosomes. This fraction is 5 to 10 per cent of the total ribosomal RNA and a much higher proportion of the RNA of the 20S and 30S ribosomes present in the cell extract. The rate of incorporation of label into this fraction and into the main fractions of ribosomal RNA of 18S and 28S suggests that the small molecules are the precursors of the large molecules. Measurements of the rate of labeling of the 20, 30, and 50S ribosomes made at corresponding times indicate that ribosome synthesis occurs by concurrent conversion of small to large molecules of RNA and small to large ribosomes.