CYTOCHROME-OXIDASE SUBUNIT-III FROM ARBACIA-LIXULA - DETECTION OF FUNCTIONAL CONSTRAINTS BY COMPARISON WITH HOMOLOGOUS SEQUENCES

In this paper we report the comparison of the sequences of the cytochr ome oxidase subunit III from three different sea urchin species. Both nucleotide and amino acid sequences have been analyzed. The nucleotide sequence analysis reveals that the sea urchin sequences obey some rul es already found in mammals. The base substitution analysis carried ou t on the sequences of the three species pairs, shows that the evolutio nary dynamics of the first and the second codon positions are so slow that do not allow a quantitative measurement of their genetic distance s, thus demonstrating that also in these species the COIII gene is str ongly conserved during evolution. Changes occurring at the third codon positions indicate that the three species evolved from a common ances tor under different directional mutational pressure. The multi-alignme nt of the sea urchin proteins indicates the existence of the amino aci d sequence motif N R T that represents a possible glycosylation site. Another glycosylation site has been detected in the mammalian cytochro me oxidase subunit III, in a position slightly different. Such an anal ysis revealed, for the first time, a new functional aspect of this seq uence.