Figure 4. Evolution of archaeal primase subunits.

(a) Phyletic patterns of the PriX and arCOG04032 families (filled circles show presence and empty circles show absence, half-filled circles show truncated proteins of arCOG04032 family containing the C-terminal domain only) are superimposed over the phylogenetic tree of archaea. The tree topology is based on the phylogeny of concatenated ribosomal proteins³⁵. The asterisk indicates that some strains of Sulfolobus islandicus encode either complete or split proteins or both. (b) A hypothetical scenario for PriX origin and evolution. Colour code, domain shapes and designations are the same as in Fig. 3.