EVOLUTIONARY ANALYSIS OF SEA-URCHIN MITOCHONDRIAL TRANSFER-RNAS - FOLDING OF THE MOLECULES AS SUGGESTED BY THE NONRANDOM OCCURRENCE OF NUCLEOTIDES

Comparative analyses of the mitochondrial tRNA sequences of the sea ur chins Arbacia lixula, Paracentrotus lividus and Strongylocentrotus pur puratus revealed that conserved nucleotides may be involved in determi ning the typical L-shaped spatial conformation of tRNAs. These results shed light on the specific tertiary interactions that allow the foldi ng of the atypical mitochondrial tRNAs into a functional form. A conse nsus mitochondrial tRNA secondary structure was derived. It shows the presence of nucleotides virtually conserved only in these organisms th at represent a sort of molecular signature in sea urchins and suggests a possible physiological role. Finally, we speculate that the non-can onical structure of animal tRNAs, as well as the deviations from the u niversality of the genetic code, may be due to the reduction in size o f the metazoan mitochondrial genome, with the concomitant acquisition of new functions by the mitochondrial tRNAs.