A tentative evolutionary scenario for the protein kinases. (Heavy lines) Major branches of the rRNA-based phylogenetic tree (Pace 1997). (Circles) Points at which individual protein-kinase families are supposed to have emerged; two paralogous subfamilies of the RIO1 family are shown. (Arrows) Postulated horizontal transfer of a protein kinase gene from one lineage to another. The postulated horizontal transfer of an ABC1 family member from bacteria to M. thermoautotrophicumis not shown.