Neighbour-joining tree built with SBcas3.3 and members of the S8A subfamily of serine proteases. Members of the S8 family of serine proteases (also known as the subtilase family) have been classified into six groups: subtilisin, thermitase, proteinase K, lantibiotic peptidase, pyrolysin (grouped into the S8A subfamily in the MEROPS database) and kexin (S8B subfamily, not shown on this figure) (Siezen and Leunissen 1997). Subtilisins have been further classified into six subfamilies, namely true subtilisins, high-alkaline proteases, intracellular proteases, phylogenetically intermediate subtilisins between true subtilisins and high-alkaline proteases, high-molecular-mass subtilisins, and oxidant-stable proteases. The phylogenetic analysis shown here was done with SBcas3.3, the protease to which SBcas3.3 is the most similar (ZP_10168560: regulatory P domain of subtilisin-like proprotein convertases, Desulfobacter postgatei) and representative members of each of the S8A groups mentioned above, as well as members of the AprX subfamily (Phrommao et al. 2011) (a new group whose position inside the S8A protease family is not well defined). The Poisson correction method was used to compute evolutionary distances. The scale bar indicates the number of amino acid substitutions per site. Bootstrap values are expressed as percentages of 1,000 replications and are shown at the nodes. Only bootstrap values higher than 50% are indicated.