Occurrence of Pseudovitamin B₁₂ and Its Possible Function as the Cofactor of Cobalamin-Dependent Methionine Synthase in a Cyanobacterium Synechocystis sp. PCC6803

Abstract

To clarify the physiological function of pseudovitamin B₁₂ in cyanobacteria, we determined pseudovitamin B₁₂ contents and cobalamin-dependent methionine synthase activity in Synechocystis sp. PCC6803 grown under CoSO₄-sufficient and -limited conditions. Pseudovitamin B₁₂ and cobalamin-dependent methionine synthase activity (0.8 nmol/min/mg protein) were found in a homogenate of the Synechocystis cells grown for 10 d in the CoSO₄-sufficient medium. The cellular pseudovitamin B₁₂ contents increased significantly at the early logarithmic growth phase and thereafter decreased rapidly at the stationary phase; a similar fluctuation pattern was shown in the cobalamin-dependent methionine synthase activity. Although the CoSO₄-limited conditions did not reduce the cell growth, pseudovitamin B₁₂ contents and methionine synthase activity decreased significantly in the limited cells relative to the sufficient cells. These results indicate that the cyanobacterium Synechocystis sp. PCC6803 can synthesize pseudovitamin B₁₂ de novo and utilize it as the cofactor for cobalamin-dependent methionine synthase.