Primary structure and phylogeny of the Calvin cycle enzymes transketolase and fructosebisphosphate aldolase of Xanthobacter flavus.

Xanthobacter flavus, a gram-negative facultatively autotrophic bacterium, employs the Calvin cycle for the fixation of carbon dioxide. Cells grown under autotrophic growth conditions possess an Fe(2+)-dependent fructosebisphosphate (FBP) aldolase (class II) in addition to a class I FBP aldolase. By nucleotide sequencing and heterologous expression in Escherichia coli, genes encoding transketolase (EC 2.2.1.1.; CbbT) and class II FBP aldolase (EC 4.1.2.13; CbbA) were identified. A partial open reading frame encoding a protein similar to pentose-5-phosphate 3-epimerase was identified downstream from cbbA. A phylogenetic tree of transketolase proteins displays a conventional branching order. However, the class II FBP aldolase protein from X. flavus is only distantly related to that of E. coli. The autotrophic FBP aldolase proteins from X. flavus, Alcaligenes eutrophus, and Rhodobacter sphaeroides form a tight cluster, with the proteins from gram-positive bacteria as the closest relatives.