The 2'-5'-oligoadenylate synthetase (OAS) belongs to a nucleotidyl transferase family that includes poly(A) polymerases and CCA-adding enzymes. In mammals and birds, the OAS functions in the interferon system but it is also present in an active form in sponges, which are devoid of the interferon system. In view of these observations, we have pursued the idea that OAS genes could be present in other metazoans and in unicellular organisms as well. We have identified a number of OAS1 genes in annelids, mollusks, a cnidarian, chordates, and unicellular eukaryotes and also found a family of proteins in bacteria that contains the five OAS-specific motifs. This indicates a specific relationship to OAS. The wide distribution of the OAS genes has made it possible to suggest how the OAS1 gene could have evolved from a common ancestor to choanoflagellates and metazoans. Furthermore, we suggest that the OASL may have evolved from an ancestor of cartilaginous fishes, and that the OAS2 and the OAS3 genes evolved from a mammalian ancestor. OAS proteins function in the interferon system in mammals. This system is only found in jawed vertebrates. We therefore suggest that the original function of OAS may differ from its function in the interferon system, and that this original function of OAS is preserved even in OAS genes that code for proteins, which do not have 2'-5'-oligoadenylate synthetase activity.

Recent studies have shown that the Porifera, with the examples of the demosponges Suberites domuncula and Geodia cydonium, comprise a series of pathways found also in the immune system of Deuterostomia, such as vertebrates, but are absent in Protostomia, with insects or nematodes as examples. One pathway is the (2¢)5¢)oligoadenylate synthetase [(2-5)A synthetase] system. In the present study we show that crude extracts from tissue of S. domuncula collected from the sea display a considerable amount of (2-5)A synthetase activity; 16% of the ATP substrate is converted to the (2-5)A product, while tissue from specimens which were kept for 6 months in an aquarium shows only 1% of conversion. As aquarium animals show a lower bacterial load, those specimens were treated for the experiments with the bacterial endotoxin lipopolysaccharide (LPS); they responded to LPS with a stimulation of the (2-5)A synthetase activity. To monitor if this effect can be obtained also on the in vitro level, primmorphs which comprise proliferating and differentiating cells, were incubated with LPS. Extracts obtained from LPS-treated primmorphs also convert ATP to the (2-5)A products mediated by the synthetase. In parallel to this effect on protein level, LPS causes after an incubation period of 12 h also an increase in the steady-state level of the transcripts encoding the putative (2-5)A synthetase. It is postulated that in sponges the (2-5)A synthetase is involved in antimicrobial defense of the animals.

In vertebrates cytokines mediate innate (natural) immunity and protect them against viral infections. The cytokine interferon causes the induction of the (2P P-5P P)oligoadenylate synthetase [(2-5)A synthetase], whose product, (2P P-5P P)oligoadenylate, activates the endoribonuclease L which in turn degrades (viral) RNA. Three isoforms of (2-5)A synthetases exist, form I (40^46 kDa), form II (69 kDa), and form III (100 kDa). Until now (2-5)A synthetases have only been cloned from birds and mammals. Here we describe the cloning of the first putative invertebrate (2-5)A synthetase from the marine sponge Geodia cydonium. The deduced amino acid sequence shows signatures characteristic for (2-5)A synthetases of form I. Phylogenetic analysis of the putative sponge (2-5)A synthetase indicates that it diverged first from a common ancestor of the hitherto known members of (vertebrate) (2-5)A synthetases I, (2-5)A synthetases II and III. Moreover, it is suggested that the (2-5)A synthetases II and III evolved from this common ancestor (very likely) by gene duplication. Together with earlier results on the existence of the (2P P-5P P)oligoadenylates in G. cydonium, the data presented here demonstrate that also invertebrates, here sponges, are provided with the (2-5)A system. At present, it is assumed that this system might be involved in growth control, including control of apoptosis, and acquired its additional function in innate immune response in evolutionarily younger animals, in vertebrates.z 1999 Federation of European Biochemical Societies.

We have proved the presence of (2′‐5′)oligoadenylates [(2′‐5′)An] and oligoadenylate synthetase [(2′‐5′)An synthetase] in the marine sponge Geodia cydonium. (2′‐5′)An isolated from sponge crude extract competed with authentic (2′‐5′)An for binding to polyclonal antiserum against (2′‐5′)An. HPLC analysis revealed the presence of nucleotides eluting with molecular markers for (2′‐5′)A oligomers. The biological activity of sponge (2′‐5′)An was demonstrated by inhibiting the protein biosynthesis in rabbit reticulocyte lysate. The activity of the (2′‐5′)An synthetase, present in crude sponge extract, was found to be high compared to that in mammalian interferon‐treated cell extract. The (2′‐5′)An synthetase from sponge extract binds to poly(I) · poly(C) as does the mammalian enzyme. Western blot analysis with antibodies to recombinant rat 43‐kDa (2′‐5′)An synthetase revealed in sponge immunologically related proteins with molecular masses of approximately 110, 65, 61 and 34 kDa. We conclude, that the (2′‐5′)An system has evolved from receptors and enzymes involved in cell adhesion and/or growth control.

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