The complete nucleotide sequences of two Suberites domuncula cDNAs and one Sycon raphanus cDNA, all encoding ubiquitin, have been determined. One cDNA from S. domuncula codes for polyubiquitin with four tandemly repeated monomeric units and the second cDNA encodes
ubiquitin fused to a ribosomal protein of 78 amino acids (aa). S. domuncula possesses at least one additional polyubiquitin gene, from which the last two monomers were also sequenced. All analysed genes from S. domuncula encode identical ubiquitin proteins, with only one aa difference
(Ala 19) to the human/higher animals ubiquitin (Pro 19). Ubiquitin in S. domuncula is identical with the ubiquitin found in another Demospongia, Geodia cydonium. The cDNA from S. raphanus encodes polyubiquitin with seven tandemly repeated units. All these gene monomers
code for the same ubiquitin, which differs from the human/higher animals ubiquitin only at position 24 (Asp in Sycon, Glu in others). However, ubiquitin from S. raphanus (Calcarea) shows two aa differences (positions 19 and 24), when compared with the ubiquitin sequences from
the two Demospongiae. In a phylogenetic tree constructed by multiple sequence alignment of all sponge ubiquitin gene monomers so far identified, all monomers from the same species cluster together, with the clear exception of the monomer from S. domuncula ribosomal protein fusion gene.
This monomer branches off first from the tree and forms a separate line; this gives evidence for a very ancient split of ubiquitin‐ribosomal‐protein fusion genes from polyubiquitin encoding genes and their long separate coexistence in eukaryotes. The ubiquitin extension protein
from S. domuncula is 78 aa long, displays all characteristics of 76–81 aa long ribosomal fusion proteins and shows 78% identity in the first 73 aa with the human S27a protein. However, its C‐terminal sequence: 69‐GLTYVYKKSD‐78 is more similar to the plant
consensus (69‐GLTYVYQ/NK‐76), than to the higher animal consensus (69‐CLTYCFNK‐76). This protein isolated from a sponge, belonging to the phylogenetically oldest multicellular animals, the Porifera, branches off first from the phylogenetic tree of metazoan ubiquitin
extension proteins of the small ribosomal subunits.
Institut für Physiologische Chemie, Abteilung Angewandte Molekularbiologie, Universität, Duesbergweg 6, D-55099 Mainz, Germany 2:
Department of Molecular Genetics, Institute Rudjer Boskovic, 10000 Zagreb, Croatia