RIBOSOMAL RNA DOMAINS. “In contrast, the specific fourth-domain hypothesis drives directly from the original definition of the three domains of cellular life. These three domains, Bacteria, Archaea, and Eukaryota, are the three major trunks in the unrooted phylogenetic tree of 16S ribosomal RNA, which is topologically consistent with the phylogenies of most of the other (nearly) universal genes that encode primarily components of the translation and the core transcription machineries.”
http://onlinelibrary.wiley.com/doi/10.1111/nyas.12728/full