Amphibian newts as experimental models for studying weight gain after castration

Vertebrate animals often exhibit sexual dimorphism in body shape. In mammals, decreases in sex hormones caused by testicular castration can affect body shape and occasionally lead to pathologies such as obesity. Post-castration obesity can also be problematic for the health of companion animals, including non-mammals. In order to understand the mechanism of post-castration obesity in vertebrates other than mammals, experimental models are required. We examined whether the Iberian ribbed newt, which has recently become a popular experimental model for amphibian research, could serve as a model for analyzing changes in body shape after castration. In newts, new testes can be regenerated after removal of differentiated testes. We analyzed changes in body shape by removing the testes under conditions in which they could regenerate or conditions in which they could not regenerate. Removal of the testes reduced blood testosterone levels. The body weight and abdominal girth of the newts were increased compared with normal male newts. Transcriptome analysis of the liver showed that a set of genes related to lipid metabolism was continuously up-regulated in castrated newts. Our study suggests that changes in body shape after castration are common in vertebrates. Iberian ribbed newts are thus a suitable model for comparative studies of the long-term physiologic- and endocrine-level effects of castration.

Generation of translucent Xenopus tropicalis through triple knockout of pigmentation genes

Amphibians generally have three types of pigment cells, namely, melanophores (black and brown), xanthophores (yellow and red), and iridophores (iridescent). Single knockout of the tyr, slc2a7, and hps6 genes in Xenopus tropicalis results in the absence of melanophores, xanthophores, and iridophores, respectively. The generation of triple- knockout (3KO) X. tropicalis for these three genes could allow for observation of internal organs without sacrificing the animals, which would be transparent due to the absence of pigments. In this study, we generated 3KO X. tropicalis, which is one of the most widely used model amphibians, through crossing of a slc2a7 single-knockout frog with a tyr and hps6 double-knockout frog, followed by intercrossing of their offspring. The 3KO tadpoles had transparent bodies like the nop mutant and the frogs had translucent bodies. This translucency allowed us to observe the heart, lungs, stomach, liver, and digestive tract through the ventral body skin without surgery. After intravital staining, 3KO X. tropicalis showed much clearer fluorescent signals of mineralized tissues compared with the wild type. These 3KO X. tropicalis provide a useful mutant line for continuous observation of internal organs and fluorescent signals in the body. In particular, such 3KO frogs would revolutionize fluorescence monitoring in transgenic tadpoles and frogs expressing fluorescent proteins.

Phenotype-genotype relationships in Xenopus sox9 crispants provide insights into campomelic dysplasia and vertebrate jaw evolution

Since CRISPR-based genome editing technology works effectively in the diploid frog Xenopus tropicalis, a growing number of studies have successfully modeled human genetic diseases in this species. However, most of their targets were limited to non-syndromic diseases that exhibit abnormalities in a small fraction of tissues or organs in the body. This is likely because of the complexity of interpreting the phenotypic variations resulting from somatic mosaic mutations generated in the founder animals (crispants). In this study, we attempted to model the syndromic disease campomelic dysplasia (CD) by generating sox9 crispants in X. tropicalis. The resulting crispants failed to form neural crest cells at neurula stages and exhibited various combinations of jaw, gill, ear, heart, and gut defects at tadpole stages, recapitulating part of the syndromic phenotype of CD patients. Genotyping of the crispants with a variety of allelic series of mutations suggested that the heart and gut defects depend primarily on frame-shift mutations expected to be null, whereas the jaw, gill, and ear defects could be induced not only by such mutations but also by in-frame deletion mutations expected to delete part of the jawed vertebrate-specific domain from the encoded Sox9 protein. These results demonstrate that Xenopus crispants are useful for investigating the phenotype-genotype relationships behind syndromic diseases and examining the tissue-specific role of each functional domain within a single protein, providing novel insights into vertebrate jaw evolution.

Osteological and histological comparison of the development of the interphalangeal intercalary skeletal element between hyloid and ranoid anurans

Some frog species have a unique skeletal element, referred to as the intercalary element (IE), in the joints between the terminal and subterminal phalanges of all digits. IEs are composed of cartilage or connective tissue and have a markedly differ shape than the phalanges. IEs are highly related to the arboreal lifestyle and toe pads. The IE is found only in neobatrachian frogs among anurans, suggesting that it is a novelty of Neobatrachia. IEs are widely distributed among multiple neobatrachian lineages and are found in the suborders Hyloides and Ranoides (the two major clades in Neobatrachia). However, it is unclear whether the IEs found in multiple linages resulted from convergent evolution. Therefore, in this study, we aimed to examine how similar or different the developmental trajectories of the IEs are between Hyloides and Ranoides. To that end, we compared the osteological and histological developmental processes of the IEs of the hyloid frog Dryophytes japonicus and the ranoid frog Zhangixalus schlegelii. Both species shared the same IE-initiation site and level of tissue differentiation around the IE when it began to form in tadpoles, although the IE developments initiated at different stages which were determined by external criteria. These results suggest that similar mechanisms drive IE formation in the digits of both species, supporting the hypothesis that the IEs did not evolve convergently.

Cryo-injury procedure-induced cardiac regeneration shows unique gene expression profiles in the newt Pleurodeles waltl

BACKGROUND

Cardiac regeneration in the adult mouse is not substantial. Some vertebrates, such as newts and zebrafish, regenerate the heart throughout their lives. To understand how regenerative abilities differ among animal species, comparative research has been conducted in animals like mouse, zebrafish, and newt. For those purposes, cryo-injury is suitable as an experimental model for the pathological condition of human myocardial infarction. In fact, cryo-injury procedures are common in mouse and zebrafish.

RESULTS

In the present study, we induced cryo-damage on the ventricle in Iberian ribbed newts using a liquid nitrogen-chilled probe. We observed that the injured area recovered within 8 weeks, with remodeling of scar tissue and proliferation of cardiomyocytes. We investigated the subsequent recovery of cryo-injured and amputated tissues by comparative analysis of the gene expression profiles following these two procedures.

CONCLUSION

Notably, we established a cryo-injury procedure for the newt and confirmed that regeneration of the cryo-damaged myocardial tissue is achieved by changes in gene expression that are milder than those observed in the amputation model. Our results suggest that the cryo-injury method is suitable for comparing the process of cardiac regeneration in the newt with that in other animal models. This article is protected by copyright. All rights reserved.

Photoperiod-independent testicular development in the model newt Pleurodeles waltl

Urodele amphibian newts have unique biological properties in male gametogenesis, in addition to their extreme regenerative capacity. Male newts are able to regenerate new testes even after reaching sexual maturity and can possess multiple testes. Notably, these animals maintain primordial germ cell-like cells in a tissue adjacent to the testis. Spermatogenesis proceeds while synchronizing in a region-specific manner in the testis. However, the newt species that have been used most commonly require 2-3 years to achieve sexual maturity, and spermatogenesis in these species shows seasonality. These traits have restricted the use of newts for studies on testicular development and spermatogenesis, and testis development in newts remains poorly characterized. Recently, the Iberian ribbed newt Pleurodeles waltl has been established as an emerging model organism. P. waltl reaches sexual maturity more quick after birth than do other newts and is capable of breeding year-round. Thus, P. waltl is expected to serve as an appealing experimental model for studying the mechanisms of male gametogenesis in the urodeles. In the present study, we use P. waltl to describe the entire developmental process of the newt testis from primordial gonad to maturity. Notably, the mature testes show synchronized progression of spermatogenesis along the anteroposterior axis. Additionally, we demonstrate that the process of spermatogenesis in P. waltl proceeds irrespective of day length. Our results show that P. waltl newts are a suitable model for investigating the process of testicular development. We also expect that these results will be useful for the maintenance of P. waltl bioresources.

A unique role of thyroid hormone receptor β in regulating notochord resorption during Xenopus metamorphosis

Tail resorption during anuran metamorphosis is perhaps the most dramatic tissue transformation that occurs during vertebrate development. Earlier studies in highly related anuran species Xenopus laevis and Xenopus tropicalis have shown that thyroid hormone (T3) receptor (TR) plays a necessary and sufficient role to mediate the causative effect of T3 on metamorphosis. Of the two known TR genes in vertebrates, TRα is highly expressed during both premetamorphosis and metamorphosis while TRβ expression is low in premetamorphic tadpoles but highly upregulated as a direct target gene of T3 during metamorphosis, suggesting potentially different functions during metamorphosis. Indeed, gene knockout studies have shown that knocking out TRα and TRβ has different effects on tadpole development. In particularly, homozygous TRβ knockout tadpoles become tailed frogs well after sibling wild type ones complete metamorphosis. Most noticeably, in TRβ-knockout tadpoles, an apparently normal notochord is present when the notochord in wild-type and TRα-knockout tadpoles disappears. Here, we have investigated how tail notochord resorption is regulated by TR. We show that TRβ is selectively very highly expressed in the notochord compared to TRα. We have also discovered differential regulation of several matrix metalloproteinases (MMPs), which are known to be upregulated by T3 and implicated to play a role in tissue resorption by degrading the extracellular matrix (ECM). In particular, MMP9-TH and MMP13 are extremely highly expressed in the notochord compared to the rest of the tail. In situ hybridization analyses show that these MMPs are expressed in the outer sheath cells and/or the connective tissue sheath surrounding the notochord. Our findings suggest that high levels of TRβ expression in the notochord specifically upregulate these MMPs, which in turn degrades the ECM, leading to the collapse of the notochord and its subsequent resorption during metamorphosis.

Homeotic transformation of tails into limbs in anurans

Anuran tadpoles can regenerate their tails after amputation. However, they occasionally form ectopic limbs instead of the lost tail part after vitamin A treatment. This is regarded as an example of a homeotic transformation. In this phenomenon, the developmental fate of the tail blastema is apparently altered from that of a tail to that of limbs, indicating a realignment of positional information in the blastema. Morphological observations and analyses of the development of skeletal elements during the process suggest that positional information in the blastema is rewritten from tail to trunk specification under the influence of vitamin A, resulting in limb formation. Despite the extensive information gained from morphological observations, a comprehensive understanding of this phenomenon also requires molecular data. We review previous studies related to anuran homeotic transformation. The findings of these studies provide a basis for evaluating major hypotheses and identifying molecular data that should be prioritized in future studies. Finally, we argue that positional information for the tail blastema changes to that for a part of the trunk, leading to homeotic transformations. To suggest this hypothesis, we present published data that favor the rewriting of positional information.

Thyroid Hormone Receptor α- and β-Knockout Xenopus tropicalis Tadpoles Reveal Subtype-Specific Roles During Development

Thyroid hormone (TH) binds TH receptor α (TRα) and β (TRβ) to induce amphibian metamorphosis. Whereas TH signaling has been well studied, functional differences between TRα and TRβ during this process have not been characterized. To understand how each TR contributes to metamorphosis, we generated TRα- and TRβ-knockout tadpoles of Xenopus tropicalis and examined developmental abnormalities, histology of the tail and intestine, and messenger RNA expression of genes encoding extracellular matrix-degrading enzymes. In TRβ-knockout tadpoles, tail regression was delayed significantly and a healthy notochord was observed even 5 days after the initiation of tail shortening (stage 62), whereas in the tails of wild-type and TRα-knockout tadpoles, the notochord disappeared after ∼1 day. The messenger RNA expression levels of genes encoding extracellular matrix-degrading enzymes (MMP2, MMP9TH, MMP13, MMP14, and FAPα) were obviously reduced in the tail tip of TRβ-knockout tadpoles, with the shortening tail. The reduction in olfactory nerve length and head narrowing by gill absorption were also affected. Hind limb growth and intestinal shortening were not compromised in TRβ-knockout tadpoles, whereas tail regression and olfactory nerve shortening appeared to proceed normally in TRα-knockout tadpoles, except for the precocious development of hind limbs. Our results demonstrated the distinct roles of TRα and TRβ in hind limb growth and tail regression, respectively.

Vitamin A induced homeotic hindlimb formation on dorsal and ventral sides of regenerating tissue of amputated tails of Japanese brown frog tadpoles

When anuran tadpoles are treated with vitamin A after tail amputation, hindlimb-like structures can be generated instead of the lost tail part at the amputation site. This homeotic transformation was initially expected to be a key to understanding the body plan of vertebrates. Unfortunately, homeotic limb formation has been reproduced in only some Indian frog species and a European species, but not in experimental anurans such as Xenopus laevis or Rana catesbeiana. Consequently, this fascinating phenomenon has not been well analyzed, especially at the molecular level. In addition, the initial processes of ectopic limb development are also unclear because morphological changes in the early phases have not been analyzed in detail. In this study, we report the induction of homeotic transformation using Japanese brown frogs and present a detailed morphological analysis. Unexpectedly, the ectopic limbs developed not only at the ventral sites, but also at the dorsal sites of the tail regenerates of vitamin A-treated tadpoles. The relationship between position and axial orientation of ectopic limbs suggested the double duplication of positional value order along the rostral-caudal axis and the dorsal-ventral axis of the tail regenerates.

Expression of the amelogenin gene in the skin of Xenopus tropicalis

Anuran skin contains a calcified dermal layer, referred to as the Eberth-Kastschenko (EK) layer, which is found between the stratum spongiosum and the stratum compactum. Although it is established that some anuran species possess the EK layer, little is known about this layer from the standpoint of evolutionary and developmental biology. We conducted a morphological analysis by staining the dorsal skin from many species with alizarin red S to investigate the calcified layer. This layer was observed in all of the anurans tested, as well as in fishes and one species of caecilian with dermal scales, but not in urodeles, amniotes, or a scaleless caecilian. All of the investigated species with dermal scales exhibited a calcified layer in their dermis, while the anurans showed the EK layer, but no scales. We also analyzed the expression of genes related to scale formation (sparc, mmp9, and mmp2) in the dorsal skin of X. tropicalis. These genes were highly expressed at the metamorphic climax stage, which preceded the deposition of calcium. Furthermore, we examined the gene expression profile of amelogenin, the major protein found in the enamel matrix of the developing tooth. In X. tropicalis, amelogenin was upregulated in the skin at the climax stage and was expressed in the adult dermis at a high level. These data provide the first experimental evidence of the expression of amelogenin in the skin. These findings will lead to a better understanding of the developmental formation of the EK layer and the function of amelogenin.

A novel Xenopus laevis larval keratin gene, xlk2: its gene structure and expression during regeneration and metamorphosis of limb and tail

A novel cytokeratin (CK) gene, xlk2, was cloned from a cDNA library prepared from regenerating limbs of Xenopus larvae. The deduced amino acid sequence indicated that its product, XLK2, is a 48 kDa type I (acidic) CK and has a high similarity to CK13, 15, and 19 with the highest homology (58%) to mouse CK15. The gene of xlk2 exclusively expressed in basal cells of the bi-layered larval epidermis, but not in other cells in larvae and not in other periods of life. Its expression was down-regulated during spontaneous and thyroid hormone-induced metamorphosis. The basal cells of the apical epidermal cap (AEC) formed on the regenerate of larval limbs terminated the expression of xlk2, whereas those of the adjacent normal epidermis continued to express it. The AEC-basal cells did not re-express the gene in the regenerate. In contrast, the basal cells of the tail regenerate also once terminated the expression of xlk2, but was able to re-express xlk2 later, supporting a notion that the "de-differentiated" basal cells of the tail epidermal regenerate re-differentiate into larval normal epidermal cells.

Expression of helix-loop-helix type negative regulators of differentiation during limb regeneration in urodeles and anurans

The urodele is capable of regenerating its limb by forming a blastema even in the adult. By contrast, the anuran, which is phylogenetically close to the urodele, loses this ability during metamorphosis and forms blastema-like tissues that develop only into a spike-like structure in the adult. In order to compare the molecular mechanism of the formation and maintenance of the blastema between the urodele and anuran, the genes encoding helix-loop-helix (HLH) type negative regulators of differentiation were characterized for both the Japanese newt, Cynops pyrrhogaster, and African clawed frog, Xenopus laevis. Cynops homologs of Id2, Id3, and HES1 and Xenopus Id2 were identified. To learn the roles of these genes in regeneration, their expression was examined. The expression of Id2 and Id3 was low in unamputated limbs, but was up-regulated in blastemas of both adult newt and Xenopus. Interestingly, transcripts of the two Id genes showed specific localizations in the blastema and the expression patterns were very similar in both species through the early to medium bud stage. Id2 was expressed predominantly in the blastemal epidermis, and Id3 was expressed equally in the blastemal epidermis and mesenchyme including cells in precartilage condensations. HES1 expression was up-regulated in the newt blastemal epidermis. It was thought that the up-regulation of these genes in the epidermis was related to the proliferation of the cells and that increased expression of these genes in the mesenchyme was related to the undifferentiated state of the blastemal cells. These results and considerations strongly suggested that the state of differentiation is similar in the early to medium bud blastema of both urodeles and anurans. The expression of Id3 remained high through to the digits stage in newts. In contrast, its expression in Xenopus decreased in spike-like regenerates, which correspond to palette-digits stage of newt regenerates. From these results, it was suggested that the blastema redifferentiates earlier in the frog than in the newt, and therefore the timing of redifferentiation of the cartilage is crucial for complete regeneration.

Structural diversity in the alpha 2-->8-linked polysialic acid chains in salmonid fish egg glycoproteins. Occurrence of poly(Neu5Ac), poly(Neu5Gc), poly(Neu5Ac, Neu5Gc), poly(KDN), and their partially acetylated forms

alpha 2-->8-Linked polysialic acid (polySia) chains terminate O-linked oligosaccharide chains on Salmonidae fish egg polysialoglycoproteins (PSGPs). Expression of these surface PSGPs are developmentally regulated and the polySia epitope is functionally implicated in a number of distinct species-specific cell-cell recognition events during fertilization and early embryogenesis. To better understand the functional diversity of these PSGPs, structural studies of the polySia chains isolated from three genera and eight species of Salmonidae fish eggs were carried out by chemical, immunochemical, enzymatic, and 1H NMR methods. A remarkable degree of structural diversity was found, including differences in the N-acyl groups, i.e. N-acetylneuraminic acid (Neu5Ac) or N-glycolylneuraminic acid (Neu5Gc), and in the presence of either O-acetyl substitution at C4, C7, or C9 or O-lactyl substitution at C9. The presence of heteropolymers containing both Neu5Ac and Neu5Gc residues was also an unexpected finding. Accordingly, the different forms of alpha 2-->8-linked homo- and heteropolymers of these polySia structures include: poly(Neu5Ac), poly(Neu5Gc), poly(Neu5,chi Ac2), poly(Neu5Gc chi Ac), poly(Neu5Ac, Neu5Gc), poly-(Neu5Ac, Neu5,chi Ac2), poly(Neu5Ac, Neu5Gc chi Ac), poly(Neu5Gc,Neu5,chi Ac2), and poly(Neu5Gc, Neu5-Gc chi Ac), where chi represents the site of acetylation at carbon atom 4, 7, or 9. The significance of this new structural information, together with our recent finding of alpha 2-->8-linked polydeaminoneuraminic acid, poly(KDN), in the rainbow trout egg vitelline envelope, is that it demonstrates the natural occurrence of multiple forms of alpha 2-->8-linked polySia chains in Salmonidae fish glycoproteins that have not been previously described. The results also predict that a remarkable array of polysialylated glycoconjugates is yet to be discovered in animals other than teleost fishes.

Intramolecular stacking association and conformation properties of a 'cap' structure, m7G5'pppUm, and the related model compounds

The stacking equilibrium quotient of the m7G5'pppUm unit, which occurs as the 5'-terminal "cap" of certain eukaryotic mRNA's, was determined by temperature-dependent difference spectrophotometry as Kstack = 1.82 at 25 degrees and pH 5. In order to evaluate the contribution of different structural modifications to the net stabilization of the cap structures of mRNA, a variety of compounds related to m7G5'pppUm were synthesized and their stacking properties were studied by the same method and compared. The results are summarized as: (1) Introduction of a methyl group into N-7 of G residue results in an increase in base stacking. (2) Methylation at 2'-OH of U residue also stabilizes the stacked structure of G-containing dimers, but it does not influence stacking interaction in m7G-containing dimers. (3) The effect of different types of internucleotide linkages on the order of stacking tendencies is: N5'ppN' greater than N5'pppN' greater than NpN'. UV hypochromicity and CD spectral measurements of the relevant dimers were also conducted, and the hypochromicity values and CD spectra of dimers in their stacked conformation were estimated by making use of the determined Kstack values. The results indicate that, while 2'-O-methylation exerts very little effect on the stacked conformation of the dimers, methylation at N-7 and the nature of the internucleotide linkage strongly influence the stacked conformation, thereby forming unusual left-handed conformations in m7G5'pppU(m), m7G5'ppU(m), and G5'ppU(m).

Simple model to account for the deoxy-versus ribodimer stacking quotient data. Estimation of apparent and intrinsic equilibrium quotients for intramolecular stacking association of purine deoxy- and ribodinucleoside monophosphates

Stacking equilibrium quotients were evaluated for three purine deoxyribo-dimers, d-ApA, d-m6Apm6A and d-GpG, using procedures developed in earlier studies, and compared with those for the corresponding ribo-dimers. It was shown that the deoxyribo-dimers were apparently more strongly stacked than the corresponding ribo-dimers at 25 degrees C. To explain the observed difference in stability (as measured by the apparent stacking quotient) in the two series of dimers a new extended two-state model is advanced, that the differences in the apparent stacking quotients are due to a limited availability of the number of the sugar-ring puckering modes in the stacked ribo-dimers as compared with the case in the stacked deoxyribo-counterparts. The intrinsic stacking quotients, which are interrelated with the apparent stacking quotients by the microscopic sugar puckering equilibrium constants, were also evaluated for the purine deoxy- and ribo-dimers, and were found to be the same in any pair of these dimers.

Determination of microscopic ionization constants and microscopic stacking equilibrium quotients of adenylyl(3' → 5')adenosine

Using the basic ionization constants for a pair of isomers, m1ApA and Apm1A, and the measured values for the overlapping pK values of ApA, the microscopic ionization constants and intramolecular stacking quotients for the monoprotonated ApA were estimated. The results indicate that, in contrast to the case of GpG, ApA did not exhibit preferential protonation on either site of 3'- and 5'-linked nucleoside bases and neither enhanced nor diminished stacking was observed for ApA and ApA as compared to ApA.

Hydrogen exchange kinetics of nucleic acids. Double and triple helices with Hoogsteen-type basepairs

The kinetics of hydrogen-tritium exchange reaction have been followed by a Sephadex technique of a double-helical poly(ribo-2-methylthio-adenylic acid) . poly(ribouridylic acid) complex with the Hoogsteen-type basepair. Only one hydrogen in every 2-methylthio-adenine . uracil basepair has been found to exchange at a measurably slow rate, 0.023 s-1 (at 0 degrees C), which is, however, much greater than that for a double-helix with the Watson-Crick type A . U pair. The kinetics of hydrogen-tritium exchange were also examined by triple-helical poly(rU) . poly(rA) . poly(rU) which involves both the Watson-Crick and Hoogsteen basepairings. Here, three hydrogens in every U . A. U base triplet have been found to exchange at a relatively slow rate, 0.0116 s-1 (at 0 degrees C). The kinetics of hydrogen-deuterium exchange reactions of these polynucleotide helices have also been followed by a stopped-flow ultraviolet absorption spectrophotometry at various temperatures. On the basis of these experimental results, the mechanism of the hydrogen exchange reactions in these helical polynucleotides was discussed. In the triple helix, the rate-determining process of the slow exchange of the three (one uracil-imide and two adenine-amino) hydrogens is considered to be the opening of the Watson-Crick part of the U. A. U triplet. This opening is considered to take place only after the opening of the Hoogsteen part of the triplet.

Stacking properties of a highly hydrophobic dinucleotide sequence, N6, N6-dimethyladenylyl(3' leads to 5')N6, N6-dimethyladenosine, occurring in 16--18-S ribosomal RNA

The thermal denaturation ultraviolet absorption spectra of N6,N6-dimethyladenylyl(3' leads to 5')-N6,N6-dimethyladenosine (m2(6)Apm2(6)A), which is a common sequence in 16--18-S ribosomal RNA, in aqueous buffer at pH 7 have been measured over the temperature range 3-90 degrees C. These data have been used to determine the thermodynamic quantitites associated with the intramolecular stacking equilibria. At 25 degrees C in neutral aqueous solution m2(6)Apmw(6)A exists mainly (about 81%) as a stacked form, so that the stacking interactions are stronger than those in the parent unmethylated adenylyl-(3'-5')adenosine (ApA), where about 52% is stacked. From the parameters of delta H and delta S, it is concluded that 'hidden' hydrophobic inteactions are of prime importance in the enhanced stability of m2(6)Apm2(6)A. Transphosphorylation reaction of ApA and m2(6)Apm2(6)A to form the corresponding cyclic 2',3'-phosphates has been studied. First-order rate constants at 25 degrees C for the reactions, which are base-catalyzed, have been obtained. Insertion of two methyl groups at N-6 of ApA reduces the rate of transphosphorylation. Effects of stacking on rates are discussed in the light of reaction mechanisms.

Interferon induction and its dependence on the primary and secondary structure of poly(inosinic acid).poly(cytidylic acid)

The synthetic interferon (IF) inducer rIn.rCn was modified by substituting the ribosyl residues with either their corresponding deoxy(dC or dI) or 2'-O-methyl analogues (mC or mI). The polynucleotide duplexes of these analogues are inactive as IF inducers. Circular dichroism (CD) studies revealed that, while the deoxy substitution produces significant changes in the conformation of rIn.rCn, the 2'-O-methyl substitution produces no detectable change. Biological competition experiments indicated that the methylated duplexes mIn.rCn, rIn.mCn, and mIn.mCn all compete with rIn.rCn for IF induction, while the deoxy duplexes dIn.rCn and rIn.dCn do not. These results are consistent with those predicted from the CD data. Copolymer duplexes (mI,rI)n.(mC,rC)n of varying degrees of methylation and residue clustering were also evaluated for IF induction in human fibroblasts. The IF-inducing capabilities of these duplexes correlated highly with the presence of clusters containing six or more consecutive ribosyl residues. These combined observations suggest that interaction of rIn.rCn with the cell in the induction process may occur in a biphasic manner involving first the topological recognition of a large segment of the RNA to allow for proper binding to the putative cellular receptor, followed by recognition of a much smaller region of the RNA corresponding to 6--12 consecutive ribosyl residues (0.5 to 1 helical turn) which is responsible for the triggering of the induction process.