The conformation of a conserved stem-loop structure in ribosomal RNA.

The RNA of small ribosomal subunits contains a conserved stem-loop structure near the 3' end. Characteristics for the hairpins are: (a) a nine-basepairs stem: (b) a conserved A-UU-G junction in the stem: (c) a conserved sequence Gm6(2)AM6(2)A sequence in the loop (except yeast mitochondria and mutants from bacteria). We are using UV-optics, micro-calorimetry and 500 MHz-NMR to investigate fragments of about 50 nucleotides cleaved from the 3' ends of small ribosomal subunit RNA's by bacteriocins. Our preliminary conclusions are: (1) Dimethylation of the adenines in the loop destabilizes the hairpin because of an increased stacking; (2) melting of the hairpin starts at the ends as well as in the middle at the A-UU-G junction; (3) basepair substitutions have an unexpectedly large effect on thermal stability.