Let's compare the proteins of Chlamydomonas with those of humans and the plant Arabidopsis. Which of the following is a protein that you find in all three species? C A. ODA16 protein required for eukaryotic flagella assembly. B Protochloriphylide reductase, and enzyme of the chlorophyll biosynthetic pathway. C. uS2, a protein required for DNA replication. D. Gas4, a protein required for assembly of cell walls.

To determine which protein is found in all three species—Chlamydomonas, humans, and the plant Arabidopsis—we need to consider the functions and presence of these proteins across different organisms:<br /><br />A. **ODA16**: This protein is specifically required for eukaryotic flagella assembly. While Chlamydomonas has flagella, humans have cilia (which are structurally similar), and Arabidopsis does not have flagella or cilia. Therefore, this protein may not be universally present in all three.<br /><br />B. **Protochlorophyllide reductase**: This enzyme is part of the chlorophyll biosynthetic pathway, which is specific to photosynthetic organisms like plants and algae. Humans do not perform photosynthesis, so this protein would not be present in humans.<br /><br />C. **uS2**: This protein is a component of the ribosome, specifically a ribosomal protein involved in protein synthesis, not DNA replication. Ribosomal proteins are highly conserved across species, including humans, plants, and algae, making it likely to be present in all three.<br /><br />D. **Gas4**: This protein is involved in the assembly of cell walls. While plants and algae have cell walls, humans do not, so this protein would not be present in humans.<br /><br />Based on this analysis, the most likely protein to be found in all three species is **C. uS2**, as ribosomal proteins are essential and conserved across all domains of life.