Relationships between two representative bacterial selenoprotein families and oxygen requirement of organisms containing these proteins. Organisms containing a member of a specific selenoprotein family (either Sec or Cys forms) were divided into four groups based on occurrence of the following: selenoproteins, Cys-containing homologs and the Sec trait, Cys-containing homologs and no Sec trait, and no representatives of this family. Distribution of organisms living under different oxygen conditions in each group is shown. (a) Peroxiredoxin (Prx). Peroxiredoxin homologs were identified by BLAST searches and included those containing the T/SxxU/C active site. (b) HesB-like protein. Sec+, organisms possessing the Sec-decoding trait; Sec-, organisms lacking the Sec-decoding trait.