Natural occurrence of 2',5'-linked heteronucleotides in marine sponges.

2',5'-oligoadenylate synthetases (OAS) as a component of mammalian interferon-induced antiviral enzymatic system catalyze the oligomerization of cellular ATP into 2',5'-linked oligoadenylates (2-5A). Though vertebrate OASs have been characterized as 2'-nucleotidyl transferases under in vitro conditions, the natural occurrence of 2',5'-oligonucleotides other than 2-5A has never been demonstrated. Here we have demonstrated that OASs from the marine sponges Thenea muricata and Chondrilla nucula are able to catalyze in vivo synthesis of 2-5A as well as the synthesis of a series 2',5'-linked heteronucleotides which accompanied high levels of 2',5'-diadenylates. In dephosphorylated perchloric acid extracts of the sponges, these heteronucleotides were identified as A2'p5'G, A2' p5'U, A2'p5'C, G2'p5'A and G2' p5'U. The natural occurrence of 2'-adenylated NAD(+) was also detected. In vitro assays demonstrated that besides ATP, GTP was a good substrate for the sponge OAS, especially for OAS from C. nucula. Pyrimidine nucleotides UTP and CTP were also used as substrates for oligomerization, giving 2',5'-linked homo-oligomers. These data refer to the substrate specificity of sponge OASs that is remarkably different from that of vertebrate OASs. Further studies of OASs from sponges may help to elucidate evolutionary and functional aspects of OASs as proteins of the nucleotidyltransferase family.

Evolution of the 2'-5'-oligoadenylate synthetase family in eukaryotes and bacteria.

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.

Evolutionary distribution of deoxynucleoside 5-monophosphate N-glycosidase, DNPH1.

Deoxynucleoside 5-monophosphate N-glycosidase, DNPH1 is a member of the nucleoside 2-deoxyribosyltransferase (NDT) family. This enzyme catalyzes the hydrolysis of deoxynucleoside monophosphates into free nucleobase moieties and 2-deoxyribose 5-phosphates. The DNPH1 enzymatic activity was first demonstrated in rats and then in humans. Subsequently the DNPH1 gene was identified in a variety of organisms, mainly in Metazoa. Herein, we demonstrate that despite DNPH1 genes being distributed in almost all metazoans, the occurrence of DNPH1genes is mosaic. For example, they cannot be found anywhere in the entire clade of Sauropsida or anywhere in the whole phyla of Arthropoda and Ctenophora. Even among mammals, there are organisms without functional DNPH1 protein (Camelidae and most likely Cetacea). By our knowledge, the DNPH1 gene is missing in plants, fungi and in majority of protists. Accordingly, the enzyme is apparently not of vital importance in all the branches of the Tree of Life. Surprisingly the DNPH1 gene may be found in archaea as well as in bacteria. This refers to the origin of the gene from the period before the archaea branched off from other bacteria. We show that the genomic and protein primary structures of DNPH1 are highly conserved and any modification in such a structure would result in conversion to a pseudogene, which could possibly be eliminated from the genome.

Identification of a novel member of 2H phosphoesterases, 2',5'-oligoadenylate degrading ribonuclease from the oyster Crassostrea gigas.

Several genes of IFN-mediated pathways in vertebrates, among them the genes that participate in the 2',5'-oligoadenylate synthetase (OAS)/RNase L pathway, have been identified in C. gigas. In the present study, we identified genes, which encode proteins having 2',5'-oligoadenylate degrading activity in C. gigas. These proteins belong to the 2H phosphoesterase superfamily and have sequence similarity to the mammalian A kinase anchoring protein 7 (AKAP7) central domain, which is responsible for the 2',5'-phosphodiesterase (2',5'-PDE) activity. Comparison of the genomic structures of C. gigas proteins with that of AKAP7 suggests that these enzymes originate from a direct common ancestor. However, the identified nucleases are not typical 2',5'-PDEs. The found enzymes catalyse the degradation of 2',5'-linked oligoadenylates in a metal-ion-independent way, yielding products with 2',3' -cyclic phosphate and 5'-OH termini similarly to the 3'-5' bond cleavage in RNA, catalyzed by metal-independent ribonucleases. 3',5'-linked oligoadenylates are not substrates for them. The preferred substrates for the C. gigas enzymes are 5'-triphosphorylated 2',5'-oligoadenylates, whose major cleavage reaction results in the removal of the 5'-triphosphorylated 2',3'-cyclic phosphate derivative, leaving behind the respective unphosphorylated 2',5'-oligoadenylate. Such a cleavage reaction results in the direct inactivation of the biologically active 2-5A molecule. The 2',5'-ribonucleases (2',5'-RNases) from C. gigas could be members of the ancient group of ribonucleases, specific to 2'-5' phosphodiester bond, together with the enzyme that was characterized previously from the marine sponge Tethya aurantium. The novel 2',5'-RNases may play a role in the control of cellular 2-5A levels, thereby limiting damage to host cells after viral infection.

Cloning and expression of ATP N-glycosidase from the freshwater sponge Ephydatia muelleri.

Previously we had discovered unusual enzymatic activity in the marine sponge Axinella polypoides, ATP N-glycosidase (Reintamm et al., 2003). We show here that the Ephydatia muelleri mRNA encoding protein with PNP_UDP_1 (phosphorylase superfamily) signature is the secreted ATP N-glycosidase. The functionality of the protein was established by recombinant expression in Pichia pastoris. In addition to the enzymatic domain, the full-length protein contains the N-terminal cysteine-rich domain belonging to the subfamily SCP_HrTT-1 (cd05559) of the SCP (sperm coating protein) superfamily (cl00133).

Deoxynucleoside 5-monophosphate N-glycosidase from a phylogenetically distant metazoa, sponge.

Deoxynucleoside 5-monophosphate N-glycosidase or DNPH1 (former name Rcl) is a nucleotide hydrolase whose expression in mammalian cancer tissues has been associated with its tumorigenic potential. Therefore, the enzyme has been studied principally in rat and human models. We found the corresponding gene also in the freshwater sponge Ephydatia muelleri, an animal phylogenetically very distant from mammals. Here we report the expression and characterization of the recombinant DNPH1 from E. muelleri. The ancient homolog of mammalian enzyme in a sponge showed the substrate specificity and catalytic efficiency similar to that in higher animals. E. muelleri DNPH1 is inhibited by the purine nucleotides with different numbers of 5'-phosphate groups (n = 1-4). Our results demonstrate that GTP but also dGTP are the best inhibitors, followed by all other purine nucleotides that were tested. Hence, the functioning of DNPH1 in cells where the natural ATP and GTP concentrations are much higher than those of the substrates, dNMPs, should normally be downregulated. We demonstrate for the first time the existence of biologically relevant natural inhibitors of DNPH1, namely ATP and GTP.

Expression and characterization of recombinant 2',5'-oligoadenylate synthetase from the marine sponge Geodia cydonium.

2',5'-oligoadenylate (2-5A) synthetases are known as components of the interferon-induced cellular defence mechanism in mammals. The existence of 2-5A synthetases in the evolutionarily lowest multicellular animals, the marine sponges, has been demonstrated and the respective candidate genes from Geodia cydonium and Suberites domuncula have been identified. In the present study, the putative 2-5A synthetase cDNA from G. cydonium was expressed in an Escherichia coli expression system to characterize the enzymatic activity of the recombinant polypeptide. Our studies reveal that, unlike the porcine recombinant 2-5A synthetase, the sponge recombinant protein associates strongly with RNA from E. coli, forming a heterogeneous set of complexes. No complete dissociation of the complex occurs during purification of the recombinant protein and the RNA constituent is partially protected from RNase degradation. We demonstrate that the sponge recombinant 2-5A synthetase in complex with E. coli RNA catalyzes the synthesis of 2',5'-phosphodiester-linked 5'-triphosphorylated oligoadenylates from ATP, although with a low specific activity. Poly(I).poly(C), an efficient artificial activator of the mammalian 2-5A synthetases, has only a minimal effect (an approximate two-fold increase) on the sponge recombinant 2-5A synthetase/bacterial RNA complex activity.

2',5'-oligoadenylate synthetase from a lower invertebrate, the marine sponge Geodia cydonium, does not need dsRNA for its enzymatic activity.

Recently, the presence of 2',5'-linked oligoadenylates and a high 2',5'-oligoadenylate synthetase activity were discovered in a lower invertebrate, the marine sponge Geodia cydonium. It has been demonstrated that mammalian 2-5A synthetase isozymes require a dsRNA cofactor for their enzymatic activity. Our results show that, unlike mammalian 2-5A synthetases, the 2-5A synthetase from the sponge acts in a dsRNA-independent manner in vitro. A prolonged incubation of the G. cydonium extract with a high concentration of a micrococcal nuclease had no effect on the activity of the 2-5A synthetase. At the same time, the micrococcal nuclease was effective within 30 min in degrading dsRNA needed for the enzymatic activity in IFN-induced PC12 cells. These results indicate that the 2-5A synthetase from G. cydonium may be active per se or is activated by some other mechanism. The sponge enzyme is capable of synthesizing a series of 2-5A oligomers ranging from dimers to octamers. The accumulation of a dimer in the predominant proportion during the first stage of the reaction was observed, followed by a gradual increase in longer oligoadenylates. By its product profile and kinetics of formation, the sponge 2-5A synthetase behaves like a specific isoform of enzymes of the 2-5A synthetase family.

RNA Interference-Guided Targeting of Hepatitis C Virus Replication with Antisense Locked Nucleic Acid-Based Oligonucleotides Containing 8-oxo-dG Modifications.

The inhibitory potency of an antisense oligonucleotide depends critically on its design and the accessibility of its target site. Here, we used an RNA interference-guided approach to select antisense oligonucleotide target sites in the coding region of the highly structured hepatitis C virus (HCV) RNA genome. We modified the conventional design of an antisense oligonucleotide containing locked nucleic acid (LNA) residues at its termini (LNA/DNA gapmer) by inserting 8-oxo-2'-deoxyguanosine (8-oxo-dG) residues into the central DNA region. Obtained compounds, designed with the aim to analyze the effects of 8-oxo-dG modifications on the antisense oligonucleotides, displayed a unique set of properties. Compared to conventional LNA/DNA gapmers, the melting temperatures of the duplexes formed by modified LNA/DNA gapmers and DNA or RNA targets were reduced by approximately 1.6-3.3°C per modification. Comparative transfection studies showed that small interfering RNA was the most potent HCV RNA replication inhibitor (effective concentration 50 (EC50): 0.13 nM), whereas isosequential standard and modified LNA/DNA gapmers were approximately 50-fold less efficient (EC50: 5.5 and 7.1 nM, respectively). However, the presence of 8-oxo-dG residues led to a more complete suppression of HCV replication in transfected cells. These modifications did not affect the efficiency of RNase H cleavage of antisense oligonucleotide:RNA duplexes but did alter specificity, triggering the appearance of multiple cleavage products. Moreover, the incorporation of 8-oxo-dG residues increased the stability of antisense oligonucleotides of different configurations in human serum.

Qualitative and quantitative aspects of 2-5A synthesizing capacity of different marine sponges.

2-5A synthetase is an important component of the mammalian antiviral 2-5A system. At present, the existence of 2-5A synthetase in the lowest animals, the marine sponges, has been demonstrated, although this enzyme has not been found in bacteria, yeast or plants. Here, we studied the 2-5A synthesizing capacity and the product profile of a variety of marine sponges belonging to Demospongia subclasses Tetractinomorpha and Ceractinomorpha. The 2-5A synthetase activity varied largely, in the range of four orders of magnitude, depending on the sponge species. Compared with the enzymes of the mammalian 2-5A synthetase family, the most active sponge species exhibited a surprisingly high 2-5A synthetase specific activity. Unlike the mammalian 2-5A synthetases that produce 2-5A oligomers in the presence of a double-stranded RNA activator, the 2-5A synthetase(s) from sponges were active without the addition of dsRNA. The sponge species differed in their product profiles. A novel product pool formed by Chondrosia reniformis was identified as a series of long 2-5A oligomers (up to 17-mers) with the prevalence of heptamers and octamers. The large variability of qualitative and quantitative characteristics of sponge 2-5A synthetases may refer to the occurrence of a variety of 2-5A synthetase isozymes in sponges.

Sponge (2',5')oligoadenylate synthetase activity in the whole sponge organism and in a primary cell culture.

A high (2',5')oligoadenylate (2-5A) synthetase activity was found in the marine sponge Geodia cydonium. Here we demonstrate that the 2-5A synthetase activity is present also in other sponge species although the level of the 2-5A synthetase activity varies in several magnitudes in different sponges. The 2-5A synthesizing activity was maintained in the primary culture produced from a sponge.

Intragenomic Profiling Using Multicopy Genes: The rDNA Internal Transcribed Spacer Sequences of the Freshwater Sponge Ephydatia fluviatilis.

Multicopy genes, like ribosomal RNA genes (rDNA), are widely used to describe and distinguish individuals. Despite concerted evolution that homogenizes a large number of rDNA gene copies, the presence of different gene variants within a genome has been reported. Characterization of an organism by defining every single variant of tens to thousands of rDNA repeat units present in a eukaryotic genome would be quite unreasonable. Here we provide an alternative approach for the characterization of a set of internal transcribed spacer sequences found within every rDNA repeat unit by implementing direct sequencing methodology. The prominent allelic variants and their relative amounts characterizing an individual can be described by a single sequencing electropherogram of the mixed amplicon containing the variants present within the genome. We propose a method for rational analysis of heterogeneity of multicopy genes by compiling a profile based on quantification of different sequence variants of the internal transcribed spacers of the freshwater sponge Ephydatia fluviatilis as an example. In addition to using conventional substitution analysis, we have developed a mathematical method, the proportion model method, to quantify the relative amounts of allelic variants of different length using data from direct sequencing of the heterogeneous amplicon. This method is based on determining the expected signal intensity values (corresponding to peak heights from the sequencing electropherogram) by sequencing clones from the same or highly similar amplicon and comparing hypothesized combinations against the values obtained by direct sequencing of the heterogeneous amplicon. This method allowed to differentiate between all specimens analysed.

A novel endoribonuclease from the marine sponge Tethya aurantium specific to 2',5'-phosphodiester bonds.

In the marine sponge Tethya aurantium a novel endoribonuclease was found which specifically catalyzed the degradation of 2',5'-phosphodiester linkages and was therefore named endo-2',5'-ribonuclease. This enzymatic reaction yielded 2',3'-cyclic phosphate and 5'-OH products similarly to the 3'-5' bond cleavage in RNA, catalyzed by metal-independent ribonucleases. The partially purified enzyme preparation was used for its biochemical characterization. The enzyme did not require the presence of metal ions for its activity. The novel nuclease exhibited a preference for 5'-phosphorylated 2',5'-oligoadenylates, but 2'-5' linkage in 5'-triphosphorylated hetero-oligomers or homo-dimers comprising guanylate or uridylate residues instead of adenylate was cleaved as well. The enzyme was also able to catalyze the degradation of 5'-unphosphorylated 2',5'-oligoadenylates, except for 2',5'-diadenylate, which were weaker substrates for the enzyme than the respective 5'-triphosphorylated forms. The observed substrate specificity may refer to the specific role of the enzyme in the degradation of natural 2',5'-oligoadenylates (2-5A) that function in the interferon-induced mammalian 2-5A system as allosteric regulators of ribonuclease L. They are produced by 2-5A synthetases (OAS) that are also present in sponges, the most ancient phylum of Metazoa. We suggest that the newly discovered endoribonuclease found in the marine sponge T. aurantium could be a representative of the group of 2',5'-specific ribonucleases that primarily control the cellular levels of 2',5'-oligoadenylates.

Expressed 2-5A synthetase genes and pseudogenes in the marine sponge Geodia barretti.

The 2',5'-oligoadenylate synthetases (2-5A synthetases, OAS) form a family of proteins presented in many branches of Metazoa. The phylum Porifera (sponges) contains OAS proteins which are different from those in vertebrates and form a distinct OAS subfamily. In turn, OAS proteins from different genera of Demospongia show rather low similarities in their primary structures. To ascertain divergence of the OAS genes within a particular sponge genus, we identified the OAS genes from the marine sponge Geodia barretti and compared them with those from another member of the genus Geodia, Geodia cydonium. The identity and similarity of the OAS sequences found in G. barretti with those from G. cydonium were considerably higher than identities and similarities compared with those from other sponges, 75% and 85% versus 27-30% and 42-47%, respectively. We also established the presence of a transcriptionally active polymorphic OAS pseudogene in the genome of G. barretti. The transcripts of the OAS pseudogene(s) lack several internal exons encoding necessary motifs for OAS enzymatic activity. The maintenance and further diversification of OAS gene(s) and pseudogene(s) suggest the prevalence of gene duplication events over the loss of gene duplicates in Geodia genomes during the evolution.

Sponge OAS has a distinct genomic structure within the 2-5A synthetase family.

2',5'-Oligoadenylate synthetases (2-5A synthetases, OAS) are enzymes that play an important role in the interferon-induced antiviral defense mechanisms in mammals. Sponges, the evolutionarily lowest multicellular animals, also possess OAS; however, their function is presently unclear. Low homology between primary structures of 2-5A synthetases from vertebrates and sponges renders their evolutionary relationship obscure. The genomic structure of vertebrate OASs has been thoroughly examined, making it possible to elucidate molecular evolution and expansion of this gene family. Until now, no OAS gene structure was available from sponges to compare it with the corresponding genes from higher organisms. In the present work, we determined the exon/intron structure of the OAS gene from the marine sponge Geodia cydonium and found it to be completely different from the strictly conserved exon/intron pattern of the OAS genes from vertebrates. This finding was corroborated by the analysis of OAS genes from another sponge, Amphimedon queenslandica, whose genome was recently sequenced. Our data suggest that vertebrate and sponge OAS genes have no direct common intron-containing ancestor and two (sub)types of OAS may be discriminated. This study opens new perspectives for understanding the phylogenesis and evolution of 2-5A synthetases as well as functional aspects of this multigene family.

ATP N-glycosidase - a novel ATP-converting activity from a marine sponge Axinella polypoides.

A novel nucleosidase enzymatic activity was discovered in the marine sponge Axinella polypoides. This enzyme, designated as ATP N-glycosidase, converts adenosine-5'-triphosphate into adenine and ribose-5-triphosphate. The crude extract of A. polypoides was capable of hydrolysing 25 micro mol ATP.min-1 per g wet weight of sponge. The catalytic activity of a sponge crude extract per mg total protein is comparable with specific activities of purified plant adenosine and bacterial AMP nucleosidases. The preferred substrate for the novel enzyme is ATP but any compound containing adenosine-5'-diphosphoryl fragment is also cleaved. The biochemical properties (Km, Kip, environmental requirements) of ATP N-glycosidase show similarities with previously described adenine-specific nucleosidases; however, the pattern of its biochemical characteristics does not match with that of any of those enzymes.