The presence of genes for the purine biosynthesis pathway in Crenarchaeota, Nanoarchaea, Thaumarchaeota, and Korarchaea. A schematic, taxonomy-based phylogenetic tree is provided, along with order names within the Crenarchaota. The typically bi-functional PurH protein appears twice, as PurH1 (the C-terminal AICAR formyltransferase domain) and PurH2 (the N-terminal IMP cyclohydrolase domain). Additional File 1 contains gene locus tags for each candidate gene. Symbols used: ■ denotes a gene that is a good match. □ denotes a match with some problems, as described in more detail in the text. ▲ is used to represent a cluster II PurP protein. [■] indicates that the expected gene is split into two adjacent loci. ■- ■ denotes a protein with a domain duplication. Where a "?" appears, a gene is necessary for an otherwise complete purine biosynthesis pathway to be functional, but no gene candidate could be identified with the data available.