The synthesis and kinetic behavior of deoxyribonucleic acid-like ribonucleic acid in bacteria

Abstract

A study has been made of RNA synthesis and the changes in nucleotide composition of newly formed RNA in five species of bacteria. The kinetics of incorporation of [ 14C]uracil and [ 32P]orthophosphate have also been studied. 1. 1. The composition of the RNA synthesized after very short exposures to 32P is intermediate between that of the bacterial DNA and ribosomal RNA. 2. 2. The rapidly labeled 14-S RNA fraction is heterogeneous and can be separated into 30–40 % DNA-like RNA (D-RNA) and the rest (60–70 %) ribosomal-like RNA (R-RNA). 3. 3. The changing base composition of newly formed RNA as a function of time is mainly the result of dilution of the 14-S material with an increasing proportion of ribosomal RNA. 4. 4. Kinetic studies indicate that the degradation of D-RNA provides material used in the synthesis of soluble RNA and DNA. 5. 5. In chloramphenicol, D-RNA is labeled at the normal rate, but the label is not so rapidly lost by degradation. 6. 6. These experiments lead to an estimate of about 1 % for the amount of D-RNA in exponentially growing bacteria, and approx. 2–3 min for its average lifetime. 7. 7. The number of D-RNA molecules thus estimated is one-tenth the number of ribosomes, assuming an average molecular weight of 200 000. If all the D-RNA acts as template for protein synthesis, then for a coding ratio of 3 nucleotides per amino acid the average rate of peptide-bond synthesis is about 0.4/sec/coding unit and each triplet could direct the incorporation of 75 amino acids during its average lifetime of 2.5 min.