A study of the relationship between polyribosomes and messenger RNA in Escherichia coli

Abstract

The distribution of messenger RNA among various cell fractions was examined by extracting RNA from different regions of sucrose gradients of lysates of pulse-labeled cells and annealing it to denatured DNA. Results indicate that, under any of the growth conditions studied, roughly two-thirds of the mRNA is associated with ribosomes (either as polysomes or monosomes). Hybridization studies showed that the amount of mRNA per culture in RC rel cells starved for 45 minutes of a required amino acid increased by a factor of 2 to 3 compared to an exponentially growing culture. Under these conditions, the polyribosomes were smaller than those typical of an exponential culture. When RC str cells were starved of an amino acid in the presence of chloramphenicol, both the increase in mRNA content and polyribosome distribution closely resembled that found in an amino acid-starved RC rel culture. On the other hand, when RC str strains were starved of an amino acid with no added protein inhibitor, the amount of mRNA remained constant or decreased by approximately one-third. Under these conditions, the polysome distribution remained similar to an exponential culture in most of the RC str strains studied, but shifted to monosomes in at least one strain. Addition of tetracycline led to a threefold increase in mRNA content and a rapid breakdown of polysomes. Energy-source shift-down led to a transient decrease in the amount of mRNA accompanied by a transient and uniform loss of polysomes. These observations are discussed within a scheme relating polyribosome distribution to the amount of mRNA in the cell.