Ag. Miller et Gs. Espie, PHOTOSYNTHETIC METABOLISM OF CYANATE BY THE CYANOBACTERIUM SYNECHOCOCCUS UTEX-625, Archives of microbiology, 162(3), 1994, pp. 151-157
Intact cells of the unicellular cyanobacterium Synechococcus UTEX 625
degraded exogenously supplied cyanate (as KOCN) to CO2 and NH3 in a li
ght-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 methion
ine sulfoximine, a glutamine synthetase inhibitor. Cyanate also suppor
ted photosynthetic O-2 evolution to a maximum rate of 188 mu mol O-2(m
gChl)(-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 productio
n from (KOCN)-C-13, occurred in the soluble enzyme fraction, but not i
n the thylakoid/carboxysome fraction, and was enhanced by HCO3- and in
hibited by the dianion oxalate. CO2, rather than HCO3-, was a product
of cyanate decomposition. The ability to decompose cyanate was not dep
endent upon pre-exposure of cells to cyanate to induce activity. The c
ollective results indicate that Synechococcus UTEX 625 possesses a con
stitutive, cytosolic cyanase (EC 4.3.99.1), similar in mechanism to th
at found in some species of heterotrophic bacteria. The reaction catal
yzed was: OCN- + HCO3- + 2 H+ --> 2 CO2 + NH3. In intact cells, the CO
2 produced by the action of cyanase on OCN- was either directly fixed
by the Calvin cycle enzyme ribulose-l,5-bisphosphate carboxylase/oxyge
nase, 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 fi
xation. 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.