Two independent, three-dimensional structures of yeast tRNAAsp, mainly differing by the conformation of the D loop, have been obtained from a multiple isomorphous replacement (MIR) X-ray analysis at 3.5-A resolution. The folding of the ribose-phosphate backbone is similar to that found for tRNAPhe; major differences concern the relative positioning of the acceptor and anticodon stems, and the conformation of the loops in the two molecules. Crystal packing involves self-complementary GUC anticodon interactions.

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SynopsisThe structure of yeast tRNAASp in aqueous solutions has been analyzed in the light of results obtained from Raman spectra recorded a t from 5 to 82OC and compared to those of' tRNAPhe. Firm evidence is given of a reversible conformation transition for tRNAAsp a t 20°C. This transition is observed for the first time in the tRNA series. The low-temperature conformation appears to have a more regular ribose-phosphate backbone and a more effective G base-stacking. This conformational change, which occurs essentially in the D loop, could be connected to the existence of two (A and B) crystal forms obtained depending on crystallization conditions. The melting temperatures, which are different for each base stacking in tRNAASp, lie in a range of about 70°C, much higher than for tRNAPhe. This fact is interpreted by a higher ratio of G-C base pairs in tRNAASp.

X-Ray crystal structure analysis of the two complexes formed shows that the reaction of Fe,(CO), with a cyclic dithiophosphinate occurs in two steps: firstly a complex with Fe-S-Fe and Fe-P-Fe bridges is formed which then reduces selectively the P-S thiophosphoryl bond of another dithiophosphinate molecule to give a thiophosphinite compIex in which the original heterocycle is retained. THE reduction-complexation of Pv=S compounds by organometallic substrates leading to Prn complexes is now a well established procedure.1J However, when P-S(C) bonds are present in the phosphorus compound the organometallic reagent cannot discriminate between the P ---S single and double bonds and the reduction-complexation fails3 A 9riovi it seems difficult to break the far stronger thiophosphoryl bond without affecting the weaker P-S(C) single bond. However, we have now achieved this and we describe here our preliminary results. . 1 Ph (2 1 + k ( 3 )