Expression of thepsbAGene in the Marine OxyphotobacteriaProchlorococcusspp

The oxygenic photosynthetic prokaryotesProchlorococcus marinusSS120 (CCMP1375) andProchlorococcussp. MED4 (CCMP 1378) were previously shown to exhibit different pigmentation and ecophysiological characteristics. The former strain has a much lower divinyl-Chl atobratio and is adapted to lower photon flux densities than the latter. In contrast to the cyanobacteria examined so far, both strains possess only one copy of thepsbAgene, encoding the D1 protein of photosystem II core. In acclimated steady-state cultures,psbAtranscript levels were always higher at high irradiances in both strains. Upon a shift from low to high light, thepsbAtranscript levels increased in both strains but more quickly in MED4 than in SS120. They decreased during the opposite shift. Iron-starved MED4 cells overexpressedpsbAat all assayed irradiances, suggesting that this species, representative of populations from naturally iron-depleted oceanic areas, may have developed a particular compensation mechanism. The similar effects of DCMU and DBMIB on the expression ofpsbAsuggest that light regulation ofpsbAinProchlorococcusmay be mediated by the electron transport chain. The energy state of cells could, however, also be involved in this regulation, since cultures of both strains subjected to darkness showedpsbAlevels significantly lower when glucose was added.