STRESS RESPONSES - Cyanobacteria - page 615

(1998) identifi ed salt-responsive proteins in A . torulosa by subtractive RNA hybridization approach
that provide inherent tolerance to many environmental stresses. Some of these proteins include
KdpATPase (K + scavenging enzyme) and the molecular chaperone GroEL.
The initial damage of salt stress is perceived at the level of transient inactivation of proteins of
photosystem I (PSI) and photosystem II (PSII) complexes (Allakhverdiev and Murata, 2008). The
'Z'-scheme of photosynthesis with the photosynthetic electron transport chain and the location
of proteins up to the level of reduction of ferredoxin and generation of ATP is depicted in Fig. 4.
Methyl viologen and p-benzoquinone constitute the in vitro electron acceptors for PSI and PSII,
respectively whereas dichlorophenolindophenol and diphenylcarbazid serve as electron acceptors
for PSI and PSII, respectively. Because salt stress due to NaCl has both osmotic and ionic effects
(Allakhverdiev et al ., 2000a), the effects are perceived both at the level of cell membrane and thylakoid
membranes. Across the cell membrane, the entry of Na + through Na + /H + antiporter activity results
in the accumulation of Na + at the PSII complex transiently inactivating the O 2 -evolving machinery.
This slow and irreversible inactivation of PSII has been attributed to be due to the destruction of Mn
cluster that constitutes the catalytic center of O 2 -evolving complex. In cyanobacteria, the O 2 -evolving
machinery is located on the luminal side of the thylakoid membrane that is stabilized by three
extrinsic proteins, a 33-kD protein, cytochrome c-550 and PsbU (Enami et al ., 1998; Shen et al ., 1998;
Nishiyama et al ., 1999). Of these proteins, cytochrome c-550 and PsbU are loosely bound to the donor
side of the core complex of PSII (Nishiyama et al ., 1997, 1999). In presence of light, the accumulated
Figure 4: The 'Z' scheme of photosynthesis with proteins of PSI and PSII and photosynthetic electron transport chain with
probable locations of electron acceptors from PSI (MV, methyl violgen) and PSII (PBQ, p-benzoquinone) and diphenylcarbazid
(DPC) and dichlorophenol indophenol (DCPIPH 2 ) as electron donors for PSII and PSI, respectively. Q A and Q B are quinones,
Phaeo=Phaeophytin, PQ=Plastoquinone; P 680 and P 700 are Chlorophyll a molecules. (Molecular machinery involved in the
'Z' scheme of photosynthesis is depicted in Figure 9 of Chapter 2).