STRESS RESPONSES - Cyanobacteria - page 635

concentration after long term acclimation (37 proteins). However, for 40% of the proteins that were
accumulated in salt acclimated Synechocystis cells, gene induction was not observed in the DNA
microarray experiments, thereby indicating the involvement of post-transcriptional regulation in
salt acclimation.
A comparison of the proteome with the transcriptome (mRNA levels of genes expressed after
salt shock of earlier studies) revealed that 89% of the proteins so induced shortly after salt shock
were also supported by the levels of mRNAs. The changes induced by salt stress in the proteins of
plasma membranes were monitored by another proteomic screening performed by Huang et al . (2002,
2006). Differential and 2-DE protein profi les led to the identifi cation of 109 proteins corresponding
to 66 different gene products, of which the production of 20 of them was induced under salt stress.
Half of these proteins related to periplasmic binding proteins belonging to the category of ABC-
transporters or hypothetical proteins. Few of the gene products expressed at highest level are those
of Fut A1 (an iron-binding protein) and Vipp 1 (for vesicle-inducing protein in plastids1) which
have been suggested to be involved in protection of PSII and in thylakoid membrane formation,
respectively. Other proteins identifi ed in the plasma membrane of Synechocystis sp. strain PCC 6803
belong to the category of proteases (regulatory subunit of ATP-dependent Clp protease, the Hho-
protease, a Hly protein), a protein belonging to CheY superfamily and two subunits of ATP synthase.
At least there were 10 newly identifi ed proteins belonging to hypothetical proteins that have no
sequence similarities with any protein of known function. One of the most signifi cant fi ndings is the
identifi cation of a GG-binding protein of the ABC-type transporter located on plasma membrane.
These observations are consistent with the earlier fi ndings on salt-enhanced gene expression (Mikkat
and Hagemann, 2000; Marin et al ., 2004).
The ability of many cyanobacterial strains to grow almost from neutral pH to alkaline range (of
pH 10.0 to 11.0) makes them versatile not only to adjust to variable pH conditions but also to adjust
to the concentrations of dissolved carbon dioxide/bicarbonate levels. Since the photoautotrophic
growth of Synechocystis sp. strain PCC 6803 is similar at pH values ranging from 7.5 to 10.0, the
probable changes that occur during a shift from pH 7.5 to pH 10.0 are worth following. Intracellular
pH levels are also maintained consistent with the external pH. Accordingly, compartmentalization
of pH in the vicinity of internal membranes and the cytosol has been observed. Thylakoid lumen
has a pH that is ~2 units lower than the pH of the cytosol (Belkin and Packer, 1988). Any change in
external pH by 2 units will increase internal pH by ~0.2 unit (Belkin and Packer, 1988; Ritchie, 1991).
The isolation and characterization of a number of pH-sensitive PSII mutants of Synechocystis sp.
strain PCC 6803 that were able to grow at alkaline pH (pH10.0) but not at pH 7.5 revealed that these
mutants were defi cient in either PsbO or PsbV lumen proteins (Eaton-Rye et al ., 2003; Summerfi eld
et al ., 2005). Keeping these in view, Summerfi eld and Sherman (2008) studied global transcriptional
response during a shift of Synechocystis cells from pH 7.5 to pH 10.0. Majority of the genes that
were up-regulated during such a shift are those pertaining to the processes of photosynthesis and
respiration. Other categories of proteins up-regulated pertain to the regulatory functions, transport
and binding proteins. Similar studies on the proteomic analysis of plasma membranes of high
pH-stressed cells of Synechocystis sp. strain PCC 6803 confi rmed the up-regulation of transport
and binding proteins of ABC transporters which included three phosphate transport proteins.
Other categories of genes up-regulated during high pH stress belonged to cell division, signalling,
photosynthesis and respiration (Zhang et al ., 2009). One of the probable mechanisms suggested for
maintaining pH homeostasis is due to accumulation of acetolactate by the cells of Synechocystis sp.
strain PCC 6803 under alkaline conditions (Maestri and Joset, 2000).