Characterisation of the slr1212 genomic region of the freshwater cyanobacterium Synechocystis sp. PCC 6803
Synechocystis sp. pee 6803 is a unicellular, freshwater cyanobacterium. Its dependence upon light to support its photoautotrophic lifestyle increases the importance of environmental sensing mechanisms to be able to maximise lightharvesting whilst avoiding the harmful effects of light-mediated cell damage. The sequencing of the Synechocystis sp. pee 6803 genome in 1996 now allows the identification of genes that encode putative proteins with roles in sensing the environment. Two such open reading frames, slr1212 and slrI213, were identified from the genome following computer analysis of the protein sequences. These two proteins encode a putative two-component signal transduction system with a role in sensing the environment. SIr1212 possesses homology to (i) the binding domain of ethylene receptors of higher plants, (ii) P AS/P AC domains, potentially involved in ligand binding and protein dimerisation, (iii) GAF domains, which contain the chromophore binding region of plant phytochromes, and (iv) histidine kinases of two-component signal transduction systems. SIr1213 possesses homology to characterised response regulators, and contains a helix-turn-helix DNA binding motif. This study set out to characterise a physiological role for these enigmatic proteins by analysing interposon mutants. Single and double interposon mutants were generated in these open reading frames. Growth of these mutants was unaffected in different light qualities, but SIr1212 was shown to be involved in the acclimation of the Synechocystis sp. pee 6803 cells to high light irradiance as analysed by 77K fluorescence spectroscopy, which also indicated possible structural alterations in PSI reaction centres ofORF slr1212 mutants. Using laser photo acoustic spectroscopy, it was shown that Synechocystis sp. pee 6803 could release ethylene following incubation with Ace suggesting a possible ethylene biosynthetic route, though genome analysis revealed no obvious homologues of ACC oxidase, an enzyme required for conversion of Ace to ethylene in vascular plants. It is hypothesised that an ethylene signalling mechanism may be present that regulates cell responses to non-specific stress. Site-directed mutagenesis of SIr1213 that caused constitutive activation of the protein had a lethal effect in Synechocystis sp. pee 6803 cells. The mutation consisted of a substitution of the conserved aspartate residue with glutamate, thus mimicking the phosphorylated state of the protein. In summary, SIr 1212 has a role in the acclimation of cells to high light irradiance and binds ethylene, and may act in conjunction with Slr1213 to modulate these responses.