PHOTOSYNTHETIC METABOLISM OF CYANATE BY THE CYANOBACTERIUM SYNECHOCOCCUS UTEX-625

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 mu mol(mgChl)(-1)h(-1) and increased 1.7-fold in the presence of methionine sulfoximine, a glutamine synthetase inhibitor. Cyanate also supported photosynthetic O-2 evolution to a maximum rate of 188 mu mol O-2(mgChl)(-1)h(-1) at pH 8 and 30 degrees C. Cyanate decomposition in cell-fi-ee extracts, measured by mass spectrometry as (CO2)-C-13 production from (KOCN)-C-13, 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-l,5-bisphosphate carboxylase/oxygenase, leading to O-2 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.