The oxygenic photosynthetic prokaryotes Pro&loro-

coccus marinas 55120 (CCMP1375) and Prochhvcoccus sp. MED4 (CCMP 1378) were previously shown to exhibit different pigmentation and ecophysiological character-istics. The former strain has a much lower divinyl-ChZ a to b ratio and is adapted to lower photon flux densities than the latter. In contrast to the cyanobacteria exam-ined so far, both strains possess only one copy of the psbA gene, encoding the Dl protein of photosystem II core. In acclimated steady-state cultures, psbA tran-script levels were always higher at high irradiances in both strains. Upon a shift from low to high light, thepsbA transcript levels increased in both strains but more quickly in MED4 than in SS120. They decreased during the opposite shift. Iron-starved MED4 cells overex-pressed psbA at all assayed u-radiances, suggesting tha