PRIMARY STRUCTURE AND PHYLOGENY OF THE CALVIN CYCLE ENZYMES TRANSKETOLASE AND FRUCTOSEBISPHOSPHATE ALDOLASE OF XANTHOBACTER-FLAVUS

Xanthobacter flavus, a gram-negative facultatively autotrophic bacteri um, employs the Calvin cycle for the fixation of carbon dioxide. Cells grown under autotrophic growth conditions possess an Fe2+-dependent f ructosebisphosphate (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) an d 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 o f transketolase proteins displays a conventional branching order. Howe ver, the class II FBP aldolase protein from X. flavus is only distantl y related to that of E. coli. The autotrophic FBP aldolase proteins fr om X. flavus, Alcaligenes eutrophus, and Rhodobacter sphaeroides form a tight cluster, with the proteins from gram-positive bacteria as the closest relatives.