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

Abstract

SummaryTo clarify the physiological function of pseudovitamin B 12 in cyanobacteria, we determined pseudovitamin B 12 contents and cobalamin-dependent methionine synthase activity in Synechocystis sp. PCC6803 grown under CoSO 4 -sufficient and -limited conditions. Pseudovitamin B 12 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 CoSO 4 -sufficient medium. The cellular pseudovitamin B 12 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 CoSO 4 -limited conditions did not reduce the cell growth, pseudovitamin B 12 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 B 12 de novo and utilize it as the cofactor for cobalamin-dependent methionine synthase. Key Words pseudovitamin B 12 , methionine synthase, cyanobacteria, Synechocystis sp. PCC6803 Vitamin B 12 or cyanocobalamin (B 12 ) has a corrin ring in which the central cobalt atom is coordinated by tetra-pyrroles. Two axial (upper and lower) ligands also coordinate the cobalt atom. The lower ligand of B 12 molecule contains a cobalt-coordinated nucleotide with 5,6-dimethylbenzimidazole as a base. The upper ligand consists of either 5 ′ -deoxyadenosine (adenosylcobalamin) or a methyl group (methylcobalamin), which functions as the cofactor for methylmalonyl-CoA mutase (MCM, EC 5.4.99.2) and methionine synthase (MS, EC 2.1.1.13), respectively, in mammals ( 1 ).Certain bacteria can synthesize B 12 and/or other B 12 -related compounds with a variety of alternative bases (adenine, pseudovitamin B 12 ; 2-methyladenine, Factor A; and so on) in the lower ligand ( 2 , 3 ). We have previously reported that edible cyanobacteria ( Spirulina platensis , Aphanothece sacrum , Aphanizomenon fros-aquae , Nostoc commune , and Nostoc flagelliforme ) contain a substantial amount of pseudovitamin B 12 ( 4 -8 ). However, there is little information available on whether these cyanobacteria have the ability to synthesize pseudovitamin B 12 de novo, and whether pseudovitamin B 12 functions as a cofactor for certain cobalamin-dependent enzymes. Although most of these edible cyanobacteria are difficult to use as experimental organisms because of their low growth rates and little biochemical and genomic information, Synechocystis sp. PCC6803 has been studied extensively as a model of cyanobacterial species in many scientific fields (biochemistry, molecular biology, and so on).In this paper, we describe the occurrence of pseudovitamin B 12 in Synechocystis sp. PCC6803, and the effects of the addition or depletion of cobalt ion as a constituent of pseudovitamin B 12 on its cell growth, pseudovitamin B 12 contents, and cobalamin-dep...

Two distinct pools of B12analogs reveal community interdependencies in the ocean

“…SCM1, HCE1, HCA1, and PS0) also produced cobalamin (Table S1), confirming earlier suggestions based on the presence of cobalamin biosynthesis genes in Thaumarchaeota genomes (10). Like other Cyanobacteria (11,13,14), four axenic strains of marine Cyanobacteria (Prochlorococcus MED4 and MIT9313 and Synechococcus WH8102 and WH7803) produced pseudocobalamin (Table S1). In all of the cobalamin or pseudocobalamin producers, we detected compounds with β ligands Me-, Ado-, and OH-but not CN- (Table S1).…”

“…1). Previous studies have shown that instead of producing cobalamin, Cyanobacteria produce pseudocobalamin (11,13,14), an analog of cobalamin in which adenine substitutes for DMB as the α ligand (12) (Fig. 1).…”

Bacterial methionine biosynthesis

“…For example, Haemophilus influenzae possesses only metE, while L. meyeri possesses only metH (Gophna et al, 2005). A minor variation in cyanobacteria, including Synechocystis species (Tanioka et al, 2009) and Spirulina platensis (Tanioka et al, 2010), is the use of adeninylcobamide as the cofactor for the metH-encoded methionine synthase, instead of cobalamin. Adeninylcobamide differs from cobalamin in that the axial group is an adeninyl moiety, as opposed to a 5,6-dimethylbenzimidazole ribonucleotyl moiety.…”

Complementation of Cobalamin Auxotrophy in Synechococcus sp. Strain PCC 7002 and Validation of a Putative Cobalamin Riboswitch In Vivo

“…Unlike the cyanobacterium Synechocystis sp. strain PCC 6803, which also solely utilizes MetH for methionine synthesis but does not require exogenous cobalamin as a growth factor (13,14), Synechococcus sp. strain PCC 7002 is not capable of de novo cobalamin biosynthesis and thus is naturally a cobalamin auxotroph (6).…”