FIG. 9.

Purine biosynthesis. In Wigglesworthia, with the exception of the phosphoribosylglycinamide formyltransferase PurN, the complete purine biosynthetic pathway is present. In Buchnera and “Candidatus Blochmannia,” the first steps, leading from PRPP to AICAR, have been lost entirely. However, AICAR is also an intermediate of histidine biosynthesis in these two organisms, allowing purine biosynthesis by the combination of histidine and purine biosynthesis pathways. The conversion of purine intermediates “downstream” of the intermediates AMP, IMP, and XMP is not entirely clear from the data derived from the genome sequence, since some dedicated enzymes apparently are missing. However, as described in the text, the respective reactions are likely to be carried out by related enzymes, possibly as a result of an expansion of the substrate specificity of these enzymes. The light grey arrows highlight missing steps in purine biosynthesis, which are replaced by parts of the ehistidine biosynthesis pathuray in Blochmannia and Buchnera, and the dark grey arrows show the parts of the purine biosynthesis pathway which very probably can be carried out by the bacteria.