ISSN:
1432-072X
Keywords:
Key words Cyanobacteria
;
Synechococcus
;
Cyanate metabolism
;
Cyanase
;
Photosynthesis
;
CO2/HCO3
;
transport
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Abstract Intact cells of the unicellular cyanobacterium Synechococcus UTEX 625 degraded exogenously supplied cyanate (as KOCN) to CO2 and NH3 in a light-dependent reaction. NH3 release to the medium was as high as 80 μmol(mgChl)–1h–1 and increased 1.7-fold in the presence of methionine sulfoximine, a glutamine synthetase inhibitor. Cyanate also supported photosynthetic O2 evolution to a maximum rate of 188 μmol O2(mgChl)–1h–1 at pH 8 and 30° C. Cyanate decomposition in cell-free extracts, measured by mass spectrometry as 13CO2 production from KO13CN, occurred in the soluble enzyme fraction, but not in the thylakoid/carboxysome fraction, and was enhanced by HCO3 – and inhibited by the dianion oxalate. CO2, rather than HCO3 –, was a product of cyanate decomposition. The ability to decompose cyanate was not dependent upon pre-exposure of cells to cyanate to induce activity. The collective results indicate that Synechococcus UTEX 625 possesses a constitutive, cytosolic cyanase (EC 4.3.99.1), similar in mechanism to that found in some species of heterotrophic bacteria. The reaction catalyzed was: OCN– + HCO3 – + 2 H+→ 2 CO2 + NH3. In intact cells, the CO2 produced by the action of cyanase on OCN– was either directly fixed by the Calvin cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, leading to O2 evolution, or leaked into the medium where it was returned to the cell by the active CO2/HCO3 – transport systems for fixation. However, leakage of CO2 from air-grown cells was only observed when the active CO2 transport system was inhibited by darkness or the CO2 analogue carbon oxysulfide.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00314468
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