Eukaryotic signalling domain homologues in archaea and bacteria. Ancient ancestry and horizontal gene transfer

Phyletic distributions of eukaryotic signalling domains were studied using recently developed sensitive methods for protein sequence analysis, with an emphasis on the detection and accurate enumeration of homologues in bacter ia and archaea. A major difference was found between the distributions of e nzyme families that are typically found in all three divisions of cellular life and non-enzymatic domain families that are usually eukaryote-specific. Previously undetected bacterial homologues were identified for# plant path ogenesis-related proteins, Pad1, von Willebrand factor type A, src homology 3 and YWTD repeat-containing domains. Comparisons of the domain distributi ons in eukaryotes and prokaryotes enabled distinctions to be made between t he domains originating prior to the last common ancestor of all known life forms and those apparently originating as consequences of horizontal gene t ransfer events. A number of transfers of signalling domains from eukaryotes to bacteria were confidently identified, in contrast to only a single case of apparent transfer from eukaryotes to archaea. (C) 1999 Academic Press.