The size and heterogeneity of the messenger RNA associated with 70 S monosomes from down-shifted Escherichia coli

Abstract

After an energy source shift-down, Escherichia coli accumulates 70 S ribosome-mRNA complexes ("70 S monosomes"). The monosome mRNA strands are predominantly primary transcription products with purine nucleoside 5'-triphosphate and 5'-diphosphate termini present at a 1:2 ratio. The number-average chain length is 564 +/- 30 nucleotides, indicating that the population represents primarily monocistronic mRNAs. Digestions with endonucleases and exonucleases indicate that the ribosomes lie near the 5' ends of the mRNA strands and that the majority of the mRNA strands contain 5'-proximal "leader" sequences (average 10 nucleotides) outside the protective boundary of the ribosome. These data are consistent with the hypothesis that the increased functional stability of mRNA in down-shifted cells may result from protection by bound ribosomes of endonuclease-susceptible site(s) near the 5' ends of the mRNA strands.