tRNA-rRNA sequence homologies: a model for the origin of a common ancestral molecule, and prospects for its reconstruction.

A model is proposed for the early evolution of the coding mechanism. A primordial RNA embodies the functions of today's nucleic acids in a single molecule. The molecule is generated by successive rounds of self-priming and -templating. After proximity is assured by enclosure in a cell, the functions can be partitioned among more efficient specialized molecules. The prediction of sequence homologies in later forms prompted a search for matches between t- and r-RNAs. These are described. Their distributions offer clues to their origins. The existance of overlapping homologies indicates an approach to the reconstruction of an ancestral molecule.