In vivo chloroplast protein synthesis by the chromophytic alga Olisthodiscus luteus

Information on the ctDNA protein coding profile of the Chlorophyta, Rhodophyta, and Chromophyta might provide clues to the evolutionary mechanism(s) by which plants diverged into these three phylogenetic groups. The purpose of this study was to examine the ctDNA protein coding profile of the chromophytic plant Olisthodiscus luteus. Whole cells were labeled in the presence of cycloheximide, an inhibitor of cytoplasmic protein synthesis. Control experiments demonstrate that the chloroplast proteins labeled in vivo by this technique form a distinct subset of the total proteins synthesized by the cell. Approximately 50 plastid proteins (35 soluble, 15 membrane) were detected after two-dimensional gel electrophoresis and fluorography. Three ctDNA-coded proteins, the large subunit of ribulosebisphosphate carboxylase, the apoprotein of the P700-chlorophyll a-protein complex, and the "photogene" were identified. These proteins are also coded by chlorophytic ctDNA. Unexpectedly, the ctDNA of Olisthodiscus was shown to code for the small subunit of ribulosebisphosphate carboxylase. The gene for this enzyme subunit is nuclear coded in all chlorophytic plants that have been analyzed.