As the Earth’s atmosphere transitioned from anoxic to highly oxic, which occurred roughly 2 billion years ago, specific proteins and molecular machines may have evolved to assist in assembling photosynthetic complexes with oxygen-sensitive cofactors. The CGL71 tetratricopeptide repeat protein is present in thylakoid membranes and has been associated with PS I ) accumulation. We showed that a C. reinhardtii cgl71 mutant exhibited photosensitivity and highly reduced levels of PS I activity and subunit polypeptides when grown in low light in the presence of a fixed carbon source. However, the mutant displayed a dramatic increase in PS I accumulation and function under hypoxic conditions, suggesting that CGL71 functions under atmospheric O₂ conditions to prevent oxidative disruption of PS I during subunit-cofactor assembly.  We are currently trying to identify other proteins which might be critical for assembly of photosynthetic complexes under oxic conditions.