RESTRICTION IN THE CLEAVAGE ACTIVITY OF HAMMERHEAD RIBOZYMES ENSURES ONGOING EVOLUTION IN PREBIOTIC RNA WORLD

Self-cleaving infectious RNAs found in many plant viruses and viroids can also cleave in trans and form hammerhead type secondary structure. It has been observed that the cleavage site must contain the triplet GUC. Also, in other cases, the sequence XUY holds good where X = A, C, G, U and Y = A, C, U but not G. The high electronegative nature of gu anosine holds the key to its resistance to cleavage which does not all ow hybrid formation between the ribozyme and substrate strands. Guanos ine resistance to cleavage might have been the starting thrust for the evolution of a translational initiation codon from XUG. A hypothesis is proposed in this regard and its evolutionary consequences are discu ssed briefly.