Studies on the heterogeneity of messenger RNA from E. Coli

The behaviour of messenger RNA from pulse-labelled Escherichia coli cells when sedimented on a linear eoneentration gradient of sucrose depends on the mode of extraction of the cells and on the concentration of Mg 2+ ions during the sedimentation. Extraction procedures, which lessen the degradative effects of nucleolytic enzymes, give messenger RNA preparations richer in heavy fractions. When Mg 2+ ions are present during the centrifugation, messenger RNA partly associates with ribosomal RNA components and sediments with them. Chromatography on methylated serum albumin-coated Kieselguhr shows that messenger RNA is heterogeneous. The chromatographic behaviour of messenger RNA fractions depends on their molecular size and probably also on their nucleotide composition. Some fractions of messenger RNA appear to have molecular sizes which correspond to sedimentation coefficients in the range 23 to 30 s, and which are probably able to control the synthesis of the longest protein monomers. The relative proportions of the various fractions are influenced by the extraction procedure but some of them are present in every preparation and probably pre-exist in growing cells.