Possible translocation of a gene for thymidylate kinase from the chloroplast to the nuclear genome during evolution.

On the basis of the fact that in Acetabularia three out of four enzymes that are involved in deoxyribonucleotide metabolism are translated on 70 S ribosomes, while a fourth one is synthesized on 80 S ribosomes, the suggestion was made that translation on 80 S ribosomes might reflect an evolutionarily more recent situation while translation on 70 S ribosomes might be of more ancient origin. This hypothesis prompted experiments aimed at defining the site of translation of thymidylate kinase in a species closely related to Acetabularia but long ago separated from it: Batophora oerstedii. It was demonstrated that in this species, in contrast to a number of Acetabulariae, thymidylate kinase is translated on 70 S ribosomes. A possible, appealing interpretation of this finding is that during evolution the coding site for thymidylate kinase has been translocated from the chloroplast to the nuclear genome.