The Effect of a Cytidine‐to‐Uridine Transition on the Stability of Escherichia coli A19 5‐S RNA

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Abstract
Several species of 5S rRNA were isolated from E. coli A19. These molecules were separated on the basis of their differing stabilities during electrophoresis on 12% polyacrylamide gels in 7 M urea. This differing stability is shown, in 1 case, to be due to different primary sequence. The sequence of the least stable of these molecules was determined and it differed from the published sequence of E. coli A19 5 S RNA in only 1 residue, i.e., a uridine in place of a cytidine at position 92. The consequent G .cntdot. U base pair, formed in a normally highly stable G .cntdot. C-rich region, is responsible for a drastic reduction in the stability of the molecule. This instability leads to a less constrained, more compact molecule which thus migrates faster in electrophoresis under denaturing conditions. This species of 5 S RNA makes up 30% of the total 5 S RNA in the 50 S ribosomal subunits in this organism. Further structural studies were carried out using S1 nuclease digestion, sodium bisulfite modification and thermal melting analysis. All these methods indicate a 5 S RNA drastically destabilized in parts of its secondary and tertiary structure. The ability of the variant 5 S RNA to recognize and form a complex with its 50 S subunit binding proteins was examined and found to be impaired.