MG2-GERM AND LUPIN SEEDS 5S RIBOSOMAL-RNA( DEPENDENCE OF THE STRUCTURE AND THERMODYNAMICS OF WHEAT)

The formation and stability of structural elements in two 5S rRNA mole cules from wheat germ (WG) and lupin seeds (LS) as a function of Mg2concentration in solution was determined using the adiabatic different ial scanning microcalorimetry (DSC). The experimentally determined the rmodynamic parameters are compared with calculations using thermodynam ic databases used for prediction of RNA structure. The 5S rRNA molecul es which show minor differences in the nucleotide sequence display ver y different thermal unfolding profiles (DSC profiles). Numerical decon volution of DSC profiles provided information about structural transfo rmations that take place in both 5S rRNA molecules. A comparative anal ysis of DSC data and the theoretical thermodynamic models of the struc ture was used to establish a relationship between the constituting tra nsitions found in the melting profiles and the unfolding of structural domains of the 5S rRNA and stability of its particular helical elemen ts. Increased concentration of Mg2+ ions induces additional internal i nteractions stabilising 5S rRNA structures found at low Na+ concentrat ions. Observed conformational transitions suggest a structural model i n which the extension of helical region E dominates over the postulate d tertiary interaction between hairpin loops. We propose that helix E is stabilised by a sequence of non-standard pairings extending this he lix by the formation of tetra loop e and an almost total reduction of loop d between helices E and D. Two hairpin structures in both 5S rRNA molecules: the extended C-C' and the extended E-E'-E'' hairpins appea r as the most stable elements of the structure. The cooperativity of t he unfolding of helixes in these 5S rRNA molecules changes already at 2 mM Mg2+.