Different fates of the chloroplast tufA gene following its transfer to the nucleus in green algae

Abstract

Previous work suggested that the tufA gene, encoding protein synthesis elongation factor Tu, was transferred from the chloroplast to the nucleus within the green algal lineage giving rise to land plants. In this report we investigate the timing and mode of transfer by examining chloroplast and nuclear DNA from the three major classes of green algae, with emphasis on the class Charophyceae, the proposed sister group to land plants. Filter hybridizations reveal a chloroplast tufA gene in all Ulvophyceae and Chlorophyceae and in some but not all Charophyceae. One charophycean alga, Coleochaete orbicularis, is shown to contain an intact but highly divergent chloroplast tufA gene, whose product is predicted to be non-functional in protein synthesis. We propose that a copy of the tufA gene was functionally transferred from the chloroplast to the nucleus early in the evolution of the Charophyceae, with chloroplast copies of varying function being retained in some but not all of the subsequently diverging lineages. This proposal is supported by the demonstration of multiple tufA-like sequences in Coleochaete nuclear DNA and in nuclear DNA from all other Charophyceae examined.