Phosphorus-31 NMR studies of E.coli ribosomes

Phosphorus-3l nuclear magnetic resonance spectra, relaxation times and nuclear Overhauser (NOE) enhancements have been measured for E. coli ribosomes, subunits and rRNA. NOE and T₁ experiments reveal that the phosphorus relaxation in this organelle is largely dipolar in origin. Moreover these results imply the presence of internal motion within the RNA chain with a correlation time of about 3-5 x lO^-9 sec. In all cases the predominant resonance is centered at about -1.5 ppm (relative to 85% H₃ PO₄) as expected for a phosphodiester linkage where there is a large degree of double helix. The linewidth narrows by about a factor of four when the ribosomal proteins are removed indicating a substantial immobilization of the RNA when it is assembled into the ribosome. In addition to the phosphodiester resonance, ribosomes also reveal one or two narrower resonances shifted to low field by 1-4 ppm. Based on the observation that these resonances show a pH dependent chemical shift, we assign them to phosphate monoesters i.e. terminal 3′ or 5′ phosphate groups. These terminal phosphates are due to short oligomers of RNA derived from the terminus of the chain.