Maximum likelihood tree of the biotin carboxyl ligase (BPL). This tree is based on 156 representative sequences and 164 conserved sites and was arbitrarily rooted. Numbers at nodes indicate bootstrap robustness values higher than 50. Triangles correspond to collapsed groups of eukaryotes. Colors on leaves represent the affiliation of the sequences to their respective domain of life: archaea (red), bacteria (blue) and eukaryotes (green). Purple crosses correspond to sequences bearing a complete N-terminal domain and thus assumed to carry out a BirA regulatory function in addition to the BPL activity; orange circles correspond to sequences lacking the N-terminal domain and thus assumed to carry only the BPL function; orange crosses correspond to sequences that bear an N-terminal domain but lacking conserved positions important for the regulatory function.