Thylakoid membranes are typical and essential features of both chloroplasts and cyanobacteria. While they are crucial for phototrophic growth of cyanobacterial cells, biogenesis of thylakoid membranes is not well understood yet. Dark-grown Synechocystis sp. PCC 6803 cells contain only rudimentary thylakoid membranes but still a relatively high amount of phycobilisomes, inactive photosystem II and active photosystem I centers. After shifting dark-grown Synechocystis sp. PCC 6803 cells into the light, “greening” of Synechocystis sp. PCC 6803 cells, i.e. thylakoid membrane formation and recovery of photosynthetic electron transport reactions, was monitored. Complete restoration of a typical thylakoid membrane system was observed within 24 hours after an initial lag phase of 6 to 8 hours. Furthermore, activation of photosystem II complexes and restoration of a functional photosynthetic electron transport chain appears to be linked to the biogenesis of organized thylakoid membrane pairs.

GrpE proteins act as co-chaperones for DnaK heat-shock proteins. The dimeric protein unfolds under heat stress conditions, which results in impaired interaction with a DnaK protein. Since interaction of GrpE with DnaK is crucial for the DnaK chaperone activity, GrpE proteins act as a thermosensor in bacteria. Here we have analyzed the thermostability and function of two GrpE homologs of the mesophilic cyanobacterium Synechocystis sp. PCC 6803 and of the thermophilic cyanobacterium Thermosynechococcus elongatus BP1. While in Synechocystis an N-terminal helix pair of the GrpE dimer appears to be the thermosensing domain and mainly mediates GrpE dimerization, the C-terminal four-helix bundle is involved in additional stabilization of the dimeric structure. The four-helix bundle domain has a key role in the thermophilic cyanobacterium, since dimerization of the Thermosynechococcus protein appears to be mediated by the four-helix bundle domain, and melting of this domain is linked to monomerization of the GrpE protein. Thus, in two related cyanobacteria the GrpE thermosensing function might be mediated by different protein domains.

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