PHYLOGENETIC RELATIONSHIP OF UBIQUITIN REPEATS IN THE POLYUBIQUITIN GENE FROM THE MARINE SPONGE GEODIA-CYDONIUM

Ubiquitin is a 76-residue protein which is highly conserved among euka ryotes. Sponge (Porifera) ubiquitin, isolated from Geodia cydonium, is encoded by a gene (termed GCUBI) with six repeats, GCUBI-1 to GCUBI-6 . All repeat units encode the same protein (with one exception: GCUBI- 4 encodes ubiquitin with a change of Leu to Val at position 71). On th e nt level the sequences of the six repeats differ considerably. All c hanges (except in GCUBI-4) are silent substitutions, which do not affe ct the protein structure. However, there is one major difference betwe en the repeats: Codons from bath codon families (TCN and AGPy) are sim ultaneously used for the serine at position 65. Using this characteris tic the repeats were divided into two groups: Group I: GCUBI-1,3 (TCT codon) and GCUBI-5 (TCC); Group II: GCUBI-2,4,6 (AGC codon). Mutationa l analysis suggests that the sponge polyubiquitin gene evolved from an ancestral monoubiquitin gene by gene duplication and successive tande m duplications. The ancestral monoubiquitin gene most probably coded f or threonine (ACC) at position 65. The first event, duplication of the monoubiquitin gene, happened some 110 million years ago. Since sponge s from the genus Geodia are known from the Cretaceous (145 million) to recent time, it is very likely that all events in the evolution of po lyubiquitin gene occurred in the same genus. Alignment data of the ''c onsensus'' ubiquitin nt sequences of different animals (man to protozo a) reflect very well the established phylogenetic relationships of the chosen organisms and show that the sponge ubiquitin gene branched off first from the multicellular organisms.