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International Symposium on Photosynthetic Prokaryotes (2000)
Meeting Report
The meaning of salt: Stress response and other phenomena
by Kay Marin (U. Rostock)

This year, the significance of the environmental factor salt was defined by several groups. At one extreme, many looked at the direct effects of high salt and the mechanisms used by cells to cope with them. At the other, some focused on the effects of high salt as a tool to investigate other phenomena. Taken together, the many interesting examinations of the effects of high salt made for an absorbing tale.

Norio Murata presented data that underlined the meaning of salt stress for recovery after photodamage. Synechocystis is normally tolerant to light stress, but when light stress was combined with salt stress, the rapid repair of PSII was interrupted. It seems that the translation of the psaB mRNA became deranged and so Synechocystis recovered only after exposure to one third of the light intensity when the salt concentration is high. This finding led to the discrimination of stresses that induce damage (high light) from those that inhibit recovery (salt).

Hans Matthijs (University of Amsterdam, Netherlands) presented evidence for a third route of cyclic electron transfer around PSI. The best way to investigate the new route was the addition of salt to the medium in order to enhance cyclic electron transfer. The addition of inhibitors that block the known cyclic electron transfer routes left a residual activity of PSI. The electron carrier responsible for this newly recognized cyclic flow could be FNR (ferredoxin-NADP reductase). Expression of the protein is increased after the addition of salt. Furthermore, the protein possesses a special aminoterminal domain in cyanobacteria. Applying this special aminoterminal domain of the protein, a fraction of FNR is evidently bound to the thylakoid membrane, which is specifically involved in this new cyclic electron transport pathway.

Karin Jäger from the lab of Peter Wolk (Michigan State University, USA) presented a poster describing mutagenesis of Anabaena PCC 7120 with a transposon carrying a salt-inducible promoter. Using this promoter, she screened for mutants that expressed the affected genes only if they were cultivated on agar medium that contained 0.1 M NaCl. Four mutants were characterized able to grow on nitrate in the presence or absence of salt, but able to grow on atmospheric N2 only in the absence of salt. By using this promoter it should be possible to identify conditional mutants involved in heterocyst formation and heterocyst spacing, where conventional transposon-tagged mutations would be lethal.

Waldemar Surosz (University of Gdansk, Poland) focused on how high salt concentration may ameliorate the toxic influence of heavy metals towards the cyanobacterium Phormidium sp. If cells of this strain were cultivated with either of the heavy metals, cadmium or copper, growth was inhibited and ultrastructural characteristics changed markedly. The addition of salt to the medium led to a diminished toxic influence in the case of cadmium but not in the case of copper.

The direct influence of salt on the synthesis of the osmoprotectant glucosylglycerol in Synechocystis PCC 6803 was investigated by the lab of Martin Hagemann (University of Rostock, Germany). Salt shock of Synechocystis PCC 6803 led to the activation of two preformed but inactive enzymes involved in synthesis of the osmoprotectant: glucosylglycerol-phosphate synthase (GGPS) and glucosylglycerol-phosphate phosphatase (GGPP). Experiments by Jana Huckauf indicated that inactive GGPS is activated by dephosphorylation, leading to the question of what protein kinase is responsible for inactivation.

More genes involved in stress responses may become evident through more systematic approaches. Yu Kanesaki and others in the group of Norio Murata (National Institut for Basic Biology, Okazaki, Japan) discovered by using DNA microarray analysis about 100 genes from Synechocystis with differential expression after salt stress and osmotic stress (more than 3000 genes are on the chip, manufactured by TaKaRa). Beside the identification of many genes encoding hypothetical proteins and ribosomal proteins they could also identified genes encoding proteins involved in the synthesis of the osmoprotectant glucosylglycerol (ggpS, glpD) and others involved in regulatory networks, such as sigma factors (rpoD, sll0306; rpoD, sll2012) and chaperones (groEL). Using similar methods, Iwane Suzuki found a different pattern of gene expression after cold shock of Synechocystis.

Another approach is being taken by Hideya Fukuzawa and colleagues. They plan the systematic gene disruption of all genes encoded by the genome of Synechocystis PCC 6803. As the number of genome sequencing projects increase and along with it the construction of DNA microarrays, the recognition of all genes involved in the responses to high salt and other stresses becomes closer to reality.