D1 exchange and the Photosystem II repair cycle in the cyanobacterium Synechococcus

Photoinhibition in cyanobacteria results when photodynamic inactivation of the Photosystem II (PS II) D1 protein exceeds new D1 synthesis, largely without the additional down-regulation processes found in plants. Furthermore, unlike plants, the cyanobacterium Synechococcus sp. PCC 7942 contains genes coding for two different forms of the D1 protein, D1:1 and D1:2. Cells containing only DI:1 have a lower quantum yield of oxygen evolution and generally higher non-photochemical quenching than do cells containing only D1:2. D1:I is the predominant form in acclimated cells, but is transiently replaced by D1:2 after a shift to excess excitation, primarily as a result of changes in transcription. The exchange is triggered not by changes in light per se, but by treatments which cause fractional PS II reaction centre closure, including excess light, low temperature and partial inhibition of electron transport by DCMU. In contrast, DBMIB or heat shock treatments do not cause PS II closure and do not drive the exchange, even though overall electron transport is inhibited. After a temperature drop, high levels of nascent D1:2 accumulate in a novel form, D1:2*, approximately 3 kDa larger than the normal D1:2 polypeptide. D1:2* is not phosphorylated, does not appear to contribute to PS II function, and is probably an inactive precursor whose processing is retarded by chilling. As the cells acclimate to the lower temperature D1:2* is cleared in parallel with full recovery of PS II activity. Interestingly, D1:2* does not accumulate upon chilling in a mutant expressing only D1:2; apparently processing in this strain continues without an acclimation lag.