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The high light-inducible polypeptides (HLIPs) are critical for survival under high light (HL) conditions in Synechocystis PCC 6803. In this article, we determined the localization of all four HLIPs in thylakoid protein complexes and examined effects of hli gene deletion on the photosynthetic protein complexes. The HliA and HliB proteins were found to be associated with trimeric photosystem I (PSI) complexes and the Slr1128 protein, whereas HliC was associated with PsaL and TMP14. The HliD was associated with partially dissociated PSI complexes. The PSI activities of the hli mutants were 3-to 4-fold lower than that of the wild type. The hli single mutants lost more than 30% of the PSI trimers after they were incubated in intermediate HL for 12 h. The reduction of PSI trimers were further augmented in these cells by the increase of light intensity. The quadruple hli deletion mutant contained less than one-half of PSI trimers following 12-h incubation in intermediate HL. It lost essentially all of the PSI trimers upon exposure to HL for 12 h. Furthermore, a mutant lacking both PSI trimers and Slr1128 showed growth defects similar to that of the quadruple hli deletion mutant under different light conditions. These results suggest that the HLIPs stabilize PSI trimers, interact with Slr1128, and protect cells under HL conditions.

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The effects of various NaCl and sorbitol concentrations in the growth medium on polyamine content and on two enzymes of the polyamine biosynthesis pathway, arginine decarboxylase (ADC) and S-adenosyl methionine decarboxylase (SAMDC), were investigated in the unicellular cyanobacterium Synechocystis sp. PCC 6803. Synechocystis cells showed no difference in growth rate when the concentration of NaCl was raised up to 550 mM. The growth rate decreased at 300 mM sorbitol, and complete inhibition of growth occurred at concentrations of v 700 mM sorbitol. Salt stress induced a moderate increase in the total cellular polyamine content, spermine in particular. Osmotic stress caused an apparent increase in the total cellular polyamine content with a marked increase of spermidine induced by 700 mM sorbitol. Importantly, a low level of spermine, which so far has never been detected in cyanobacteria, could be found in Synechocystis sp. PCC 6803. ADC, a key enzyme for putrescine synthesis, was unaffected by salt stress but showed a six-fold increase in enzyme activity upon osmotic stress imposed by 700 mM sorbitol. SAMDC, another important enzyme for spermidine and spermine synthesis, responded to salt and osmotic stresses similarly to the pattern observed for ADC. An analysis by reverse transcription-polymerase chain reaction revealed an increase of ADC mRNA level in cells under salt and osmotic stresses. Most importantly, the increase of ADC mRNA was attributed to its slower turnover rate under both stress conditions. Interestingly, the samdc gene(s) of Synechocystis appear to be unique since comparisons with known gene sequences from other organisms resulted in no homologous sequences identified in the Synechocystis genome. ß