Evolution of Sucrose Synthesis

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Abstract
Cyanobacteria and proteobacteria (purple bacteria) are the only prokaryotes known to synthesize sucrose (Suc). Suc-P synthase, Suc-phosphatase (SPP), and Suc synthase activities have previously been detected in several cyanobacteria, and genes coding for Suc-P synthase (sps) and Suc synthase (sus) have been cloned from Synechocystissp. PCC 6803 and Anabaena (Nostoc) spp., respectively. An open reading frame in the Synechocystisgenome encodes a predicted 27-kD polypeptide that shows homology to the maize (Zea mays) SPP. Heterologous expression of this putative spp gene in Escherichia coli, reported here, confirmed that this open reading frame encodes a functional SPP enzyme. The Synechocystis SPP is highly specific for Suc-6F-P (K m = 7.5 μm) and is Mg2+ dependent (K a = 70 μm), with a specific activity of 46 μmol min−1 mg−1 protein. Like the maize SPP, theSynechocystis SPP belongs to the haloacid dehalogenase superfamily of phosphatases/hydrolases. Searches of sequenced microbial genomes revealed homologs of the Synechocystis sps gene in several other cyanobacteria (Nostoc punctiforme,Prochlorococcus marinus strains MED4 and MIT9313, andSynechococcus sp. WH8012), and in three proteobacteria (Acidithiobacillus ferrooxidans,Magnetococcus sp. MC1, and Nitrosomonas europaea). Homologs of the Synechocystis sppgene were found in Magnetococcus sp. MC1 andN. punctiforme, and of the Anabaena susgene in N. punctiforme and N. europaea. From analysis of these sequences, it is suggested that Suc synthesis originated in the proteobacteria or a common ancestor of the proteobacteria and cyanobacteria.