Basic Charges on Mammalian R bonuclease Molecules and the Ability to Attack Double-Stranded. RNA

Duplication and parallel evolution of the pancreatic ribonuclease gene (RNASE1) in folivorous non-colobine primates, the howler monkeys (Alouatta spp.)

In foregut-fermenting mammals (e.g., colobine monkeys, artiodactyl ruminants) the enzymes pancreatic ribonuclease (RNASE1) and lysozyme C (LYZ), originally involved in immune defense, have evolved new digestive functions. Howler monkeys are folivorous non-colobine primates that lack the multi-chambered stomachs of colobines and instead digest leaves using fermentation in the caeco-colic region. We present data on the RNASE1 and LYZ genes of four species of howler monkey (Alouatta spp.). We find that howler monkey LYZ is conserved and does not share the substitutions found in colobine and cow sequences, whereas RNASE1 was duplicated in the common ancestor of A. palliata, A. seniculus, A. sara, and A. pigra. While the parent gene (RNASE1) is conserved, the daughter gene (RNASE1B) has multiple amino acid substitutions that are parallel to those found in RNASE1B genes of colobines. The duplicated RNase in Alouatta has biochemical changes similar to those in colobines, suggesting a novel, possibly digestive function. These findings suggest that pancreatic ribonuclease has, in parallel, evolved a new role for digesting the products of microbial fermentation in both foregut- and hindgut-fermenting folivorous primates. This may be a vital digestive enzyme adaptation allowing howler monkeys to survive on leaves during periods of low fruit availability.

DNA melting properties of the dityrosine cross-linked dimer of Ribonuclease A

Several DNA binding proteins exist in dimeric form when bound with DNA to be able to exhibit various biological processes such as DNA repair, DNA replication and gene expression. Various dimeric forms of Ribonuclease A (RNase A) and other members of the ribonuclease A superfamily are endowed with a multitude of biological activities such as antitumor and antiviral activity. In the present study, we have compared the DNA binding properties between the RNase A monomer and the dityrosine (DT) cross-linked RNase A dimer, and checked the inhibitory effect of DNA on the ribonucleolytic activity of the dimeric protein. An agarose gel based assay shows that like the monomer, the dimer also binds with DNA. The number of nucleotides bound per monomer unit of the dimer is higher than the number of nucleotides that bind with the each monomer. From fluorescence measurements, the association constant (Ka) values for complexation of the monomer and the dimer with ct-DNA are (4.95±0.45)×10(4)M(-1) and (1.29±0.05)×10(6)M(-1) respectively. Binding constant (Kb) values for the binding of the monomer and the dimer with ct-DNA were determined using UV-vis spectroscopy and were found to be (4.96±1.67)×10(4)M(-1) and (4.32±0.31)×10(5)M(-1) respectively. Circular dichroism studies shows that the dimer possesses significant effect on DNA conformation. The melting profile for the ct-DNA-dimer indicated that the melting temperature (Tm) for the ct-DNA-dimer complex is lower compared to the ct-DNA-monomer complex. The ribonucleolytic activity of the dimer, like the monomer, diminishes upon binding with DNA.

Evolutionary and functional novelty of pancreatic ribonuclease: a study of Musteloidea (order Carnivora)

Pancreatic ribonuclease (RNASE1) is a digestive enzyme that has been one of the key models in studies of evolutionary innovation and functional diversification. It has been believed that the RNASE1 gene duplications are correlated with the plant-feeding adaptation of foregut-fermenting herbivores. Here, we characterized RNASE1 genes from Caniformia, which has a simple digestive system and lacks microbial digestion typical of herbivores, in an unprecedented scope based on both gene sequence and tissue expression analyses. Remarkably, the results yielded new hypotheses regarding the evolution and the function of Caniformia RNASE1 genes. Four independent gene duplication events in the families of superfamily Musteloidea, including Procyonidae, Ailuridae, Mephitidae and Mustelidae, were recovered, rejecting previous Mustelidae-specific duplication hypothesis, but supporting Musteloidea duplication hypothesis. Moreover, our analyses revealed pronounced differences among the RNASE1 gene copies regarding their selection pressures, pI values and tissue expression patterns, suggesting the differences in their physiological functions. Notably, the expression analyses detected the transcription of a RNASE1 pseudogene in several tissues, raising the possibility that pseudogenes are also a potential source during the RNase functional diversification. In sum, the present work demonstrated a far more complex and intriguing evolutionary pattern and functional diversity of mammalian ribonuclease than previously thought.

The solution structure and dynamics of human pancreatic ribonuclease determined by NMR spectroscopy provide insight into its remarkable biological activities and inhibition

Human pancreatic ribonuclease (RNase 1) is expressed in many tissues; has several important enzymatic and biological activities, including efficient cleavage of single-stranded RNA, double-stranded RNA and double-stranded RNA-DNA hybrids, digestion of dietary RNA, regulation of vascular homeostasis, inactivation of the HIV, activation of immature dendritic cells and induction of cytokine production; and furthermore shows potential as an anti-tumor agent. The solution structure and dynamics of uncomplexed, wild-type RNase 1 have been determined by NMR spectroscopy methods to better understand these activities. The family of 20 structures determined on the basis of 6115 unambiguous nuclear Overhauser enhancements is well resolved (pairwise backbone RMSD=1.07 A) and has the classic RNase A type of tertiary structure. Important structural differences compared with previously determined crystal structures of RNase 1 variants or inhibitor-bound complexes are observed in the conformation of loop regions and side chains implicated in the enzymatic as well as biological activities and binding to the cytoplasmic RNase inhibitor. Multiple side chain conformations observed for key surface residues are proposed to be crucial for membrane binding as well as translocation and efficient RNA hydrolysis. (15)N-(1)H relaxation measurements interpreted with the standard and our extended Lipari-Szabo formalism reveal rigid regions and identify more dynamic loop regions. Some of the most dynamic areas are key for binding to the cytoplasmic RNase inhibitor. This finding and the important differences observed between the structure in solution and that bound to the inhibitor are indications that RNase 1 to inhibitor binding can be better described by the "induced fit" model rather than the rigid "lock-into-key" mechanism. Translational diffusion measurements reveal that RNase 1 is predominantly dimeric above 1 mM concentration; the possible implications of this dimeric state for the remarkable biological properties of RNase 1 are discussed.

Cytotoxicity of Polyspermine-Ribonuclease A and Polyspermine-Dimeric Ribonuclease A

Polyspermine-ribonuclease A (PS-RNase A) and polyspermine-dimeric ribonuclease A (PS-dimeric RNase A) were prepared by cross-linking ribonuclease A or its covalently linked dimer to polyspermine (PS) using dimethyl suberimidate. The two RNase A derivatives were tested for a possible antitumor action. The in vitro and in vivo cytotoxic activity of PS-RNase A, although strong, is not higher than that known for free polyspermine. PS-dimeric RNase A, which was characterized by mass spectroscopy, titration of free amine groups, and enzymatic assays, proved instead to be a definitely more efficient antitumor agent, both in vitro and in vivo. This result could tentatively be explained in view of the importance of positive charges for ribonuclease activity, considering the higher basicity of PS-dimeric RNase A compared to that of PS-(monomeric)RNase A. It must be also taken into account that the dimeric RNase A moiety of PS-dimeric RNase A could evade the cytoplasmic ribonuclease inhibitor, which instead could trap the monomeric RNase A moiety of the other derivative. The two RNase A derivatives degrade poly(A).poly(U) under conditions where native RNase A is inactive. The results of this work demonstrate once again the importance of positive charges for the functions of mammalian pancreatic type ribonucleases in general, in particular for RNase A derivatives, and the potential therapeutic use of the ribonuclease A derivatives.

Duplication and divergence of 2 distinct pancreatic ribonuclease genes in leaf-eating African and Asian colobine monkeys

Unique among primates, the colobine monkeys have adapted to a predominantly leaf-eating diet by evolving a foregut that utilizes bacterial fermentation to breakdown and absorb nutrients from such a food source. It has been hypothesized that pancreatic ribonuclease (pRNase) has been recruited to perform a role as a digestive enzyme in foregut fermenters, such as artiodactyl ruminants and the colobines. We present molecular analyses of 23 pRNase gene sequences generated from 8 primate taxa, including 2 African and 2 Asian colobine species. The pRNase gene is single copy in all noncolobine primate species assayed but has duplicated more than once in both the African and Asian colobine monkeys. Phylogenetic reconstructions show that the pRNase-coding and noncoding regions are under different evolutionary constraints, with high levels of concerted evolution among gene duplicates occurring predominantly in the noncoding regions. Our data suggest that 2 functionally distinct pRNases have been selected for in the colobine monkeys, with one group adapting to the role of a digestive enzyme by evolving at an increased rate with loss of positive charge, namely arginine residues. Conclusions relating our data to general hypotheses of evolution following gene duplication are discussed.

Degradation of double-stranded RNA by human pancreatic ribonuclease: crucial role of noncatalytic basic amino acid residues

Under physiological salt conditions double-stranded (ds) RNA is resistant to the action of most mammalian extracellular ribonucleases (RNases). However, some pancreatic-type RNases are able to degrade dsRNA under conditions in which the activity of bovine RNase A, the prototype of the RNase superfamily, is essentially undetectable. Human pancreatic ribonuclease (HP-RNase) is the most powerful enzyme to degrade dsRNA within the tetrapod RNase superfamily, being 500-fold more active than the orthologous bovine enzyme on this substrate. HP-RNase has basic amino acids at positions where RNase A shows instead neutral residues. We found by modeling that some of these basic charges are located on the periphery of the substrate binding site. To verify the role of these residues in the cleavage of dsRNA, we prepared four variants of HP-RNase: R4A, G38D, K102A, and the triple mutant R4A/G38D/K102A. The overall structure and active site conformation of the variants were not significantly affected by the amino acid substitutions, as deduced from CD spectra and activity on single-stranded RNA substrates. The kinetic parameters of the mutants with double-helical poly(A).poly(U) as a substrate were determined, as well as their helix-destabilizing action on a synthetic DNA substrate. The results obtained indicate that the potent activity of HP-RNase on dsRNA is related to the presence of noncatalytic basic residues which cooperatively contribute to the binding and destabilization of the double-helical RNA molecule. These data and the wide distribution of the enzyme in different organs and body fluids suggest that HP-RNase has evolved to perform both digestive and nondigestive physiological functions.

Structural versatility of bovine ribonuclease A. Distinct conformers of trimeric and tetrameric aggregates of the enzyme

Lyophilization of bovine ribonuclease A (RNase A; Sigma, type XII-A) from 40% acetic acid solutions leads to the formation of approximately 14 aggregated species that can be separated by ion-exchange chromatography. Several aggregates were identified, including two variously deamidated dimeric subspecies, two distinct trimeric and two distinct tetrameric RNase A conformers, besides the two forms of dimer characterized previously [Gotte, G. & Libonati, M. (1998) Two different forms of aggregated dimers of ribonuclease A. Biochim. Biophys. Acta 1386, 106-112]. We also have possible evidence for the existence of two forms of pentameric RNase A. The two forms of trimers and tetramers are characterized by: (a) slightly different gel filtration patterns; (b) different retention times in ion-exchange chromatography; and (c) different mobilities in cathodic gel electrophoresis under nondenaturing conditions. Therefore, they appear to have distinct structural organizations responsible for a different availability of their positively charged amino acid residues. All RNase A oligomers, in particular the two distinct trimeric and tetrameric conformers, degrade poly(A).poly(U), viral double-stranded RNA and polyadenylate with a catalytic efficiency that is in general higher for the more basic species. On the contrary, the activity of the RNase A oligomers, from dimer to pentamer, on yeast RNA and poly(C) (Kunitz assay) is lower than that of monomeric RNase A.

Origin of the catalytic activity of bovine seminal ribonuclease against double-stranded RNA

Bovine seminal ribonuclease (RNase) binds, melts, and (in the case of RNA) catalyzes the hydrolysis of double-stranded nucleic acid 30-fold better under physiological conditions than its pancreatic homologue, the well-known RNase A. Reported here are site-directed mutagenesis experiments that identify the sequence determinants of this enhanced catalytic activity. These experiments have been guided in part by experimental reconstructions of ancestral RNases from extinct organisms that were intermediates in the evolution of the RNase superfamily. It is shown that the enhanced interactions between bovine seminal RNase and double-stranded nucleic acid do not arise from the increased number of basic residues carried by the seminal enzyme. Rather, a combination of a dimeric structure and the introduction of two glycine residues at positions 38 and 111 on the periphery of the active site confers the full catalytic activity of bovine seminal RNase against duplex RNA. A structural model is presented to explain these data, the use of evolutionary reconstructions to guide protein engineering experiments is discussed, and a new variant of RNase A, A(Q28L K31C S32C D38G E111G), which contains all of the elements identified in these experiments as being important for duplex activity, is prepared. This is the most powerful catalyst within this subfamily yet observed, some 46-fold more active against duplex RNA than RNase A.

Origin of dimeric structure in the ribonuclease superfamily

To enable application of postgenomic evolutionary approaches to understand the divergence of behavior and function in ribonucleases (RNases), the impact of divergent sequence on the divergence of tertiary and quaternary structure is analyzed in bovine pancreatic and seminal ribonucleases, which differ by 23 amino acids. In a crystal, seminal RNase is a homodimer joined by two "antiparallel" intersubunit disulfide bonds between Cys-31 from one subunit and Cys-32' from the other and having composite active sites arising from the "swap" of residues 1-20 from each subunit. Specialized Edman degradation techniques have completed the structural characterization of the dimer in solution, new cross-linking methods have been developed to assess the swap, and sequence determinants of quaternary structure have been explored by protein engineering using the reconstructed evolutionary history of the protein family as a guide. A single Cys at either position 32 (the first to be introduced during the divergent evolution of the family) or 31 converts monomeric RNase A into a dimer. Even with an additional Phe at position 31, another residue introduced early in the seminal lineage, swap is minimal. A hydrophobic contact formed by Leu-28, however, also introduced early in the seminal lineage, increases the amount of "antiparallel" connectivity of the two subunits and facilitates swapping of residues 1-20. Efficient swapping requires addition of a Pro at position 19, a residue also introduced early in the divergent evolution of the seminal RNase gene. Additional cysteines required for dimer formation are found to slow refolding of the protein through formation of incorrect disulfide bonds, suggesting a paradox in the biosynthesis of the protein. Further studies showed that the dimeric form of seminal RNase known in the crystal is not the only form in vivo, where a substantial amount of heterodimer is known. These data complete the acquisition of the background needed to understand the evolution of new structure, behavior, and function in the seminal RNase family of proteins.

Cross-linked trimers of bovine ribonuclease A: activity on double-stranded RNA and antitumor action

Trimers of bovine pancreatic RNase A were obtained by cross-linking native RNase A with dimethyl suberimidate. They degrade double-stranded RNA more efficiently than dimers and monomers of RNase A, and display significant cytotoxic and/or cytostatic actions against C4-I cells (a human cell line derived from squamous carcinoma of the uterus cervix). On the same cell line cross-linked dimers of RNase A appear to be ineffective.

Structural basis for the biological activities of bovine seminal ribonuclease

Bovine seminal ribonuclease (BS-RNase) is a homolog of RNase A with special biological properties that include specific antitumor, aspermatogenic, and immuno-suppressive activities. Unlike RNase A, BS-RNase is a dimer cross-linked by disulfide bonds between Cys31 of one subunit and Cys32 of the other. At equilibrium, this dimer is a mixture of two distinct quaternary forms, M = M and M x M. The conversion of M = M to M x M entails the exchange of NH2-terminal alpha-helices between subunits. Here, the cytotoxic activities of purified M x M were shown to be greater than those of purified M = M, despite extensive equilibration of M = M and M x M during the time course of the assays. Replacing Cys31 or Cys32 with a serine residue did not compromise the enzymatic activity of dimeric BS-RNase, but reduced both the fraction of M x M at equilibrium and the cytotoxicity. We conclude that the M x M form is responsible for the special biological properties of BS-RNase. Since cytosolic ribonuclease inhibitor binds tightly to monomeric but not dimeric BS-RNase and only the M x M form can remain dimeric in the reducing environment of the cytosol, we propose that BS-RNase has evolved its M x M form to retain its lethal enzymatic activity in vivo.

Human seminal ribonuclease. A tool to check the role of basic charges and glycosylation of a ribonuclease in the action of the enzyme on double-stranded RNA

Human seminal ribonuclease (a basic protein occurring in a glycosylated and in a non-glycosylated form) is very active against double-stranded RNAs (De Prisco, R., Sorrentino, S., Leone, E. and Libonati, M. (1984) Biochim. Biophys. Acta 788, 356-363). The action of the two enzyme forms on single-stranded and double-stranded substrates was studied as a function of pH and ionic strength. Results indicate (1) that glycosylation of the RNAase molecule does not affect enzyme action on single-stranded RNAs, while (2) degradation of double-stranded RNAs is moderately increased by the presence of carbohydrates in the enzyme molecule. Human seminal RNAase shows a marked helix-destabilizing activity on poly(dA-dT) X poly(dA-dT). Under various conditions, this action (1) is definitely stronger than that of bovine RNAase A, and (2) seems to be less dependent on the glycosylation than on the basicity of the enzyme protein. The remarkable activity of human seminal RNAase on double-stranded RNA may, at least partly, be related to the enzyme properties mentioned above.

A ribonuclease from human seminal plasma active on double-stranded RNA

A ribonuclease, active on single- and double-stranded RNAs, has been isolated from human seminal plasma 3-5 micrograms of enzyme were recovered per ml of seminal plasma, equivalent to 71% of total activity and a 2500-fold purification (measured with poly(A) X poly(U) as substrate) from the initial dialyzed material. Similar amounts of RNAase were found per g (wet weight) of human prostate, where the enzyme appears to be produced. Human seminal RNAase degrades poly(U) 3-times faster than poly(A) X poly(U), and poly(C) or viral single-stranded RNA about 10-times faster than poly(U). Degradation of poly(A) X poly(U), viral double-stranded RNA, and poly(A) by human seminal RNAase is 500-, 380- and 140-times more efficient, respectively, than by bovine RNAase A. The enzyme, a basic protein with maximum absorbance at 276 nm, occurs in two almost equivalent forms, one of which is glycosylated. Mr values of the glycosylated and non-glycosylated form are 21000 and 16000, respectively. The amino-acid composition of the RNAase is very similar to that of human pancreatic RNAase. The same is true for the carbohydrate content of its glycosylated form.

The amino acid sequence of human pancreatic ribonuclease

The primary structure of human (Homo sapiens) pancreatic ribonuclease has been determined by automatic sequencing of the native protein and by analysis of peptides obtained by cleavage with proteolytic enzymes, cyanogen bromide, and hydroxylamine. The following sequence was deduced: (sequence in text). Human pancreatic ribonuclease differs at 37 positions from bovine pancreatic ribonuclease. In addition the human enzyme has three more residues at the C-terminus. About half of the enzyme molecules contain carbohydrate attached to the sequence Asn-Met-Thr (34-36). Two other Asn-X-Ser/Thr sequences are carbohydrate free. Human pancreatic ribonuclease contains many positively charged residues, especially near the N-terminus, while negatively charged residues are more concentrated near the C-terminus.

Dimerization of deoxyribonuclease I, lysozyme and papain. Effects of ionic strength on enzymic activity

Transition of bovine ribonuclease A from its monomeric to a dimeric form changes the pattern of enzymic activity response to ionic strength [Sorrentino, S., Carsana, A., Furia, A., Doskocil, J., and Libonati, M. (1980) Biochim. Biophys. Acta. 609, 40-52]. To see whether this phenomenon could be common to other enzyme-substrate systems, the action of various dimeric and monomeric enzymes (ox pancreas deoxyribonuclease, hog spleen acid deoxyribonuclease, bovine seminal ribonuclease, egg-white lysozyme, and papain) on polyelectrolytic substrates has been studied under different conditions of ionic strength. Dimerization of ox pancreas deoxyribonuclease, lysozyme and papain was obtained by cross-linkage with dimethyl suberimidate. The main results of the investigation, similar to those obtained with ribonuclease A, are the following. 1. Enzyme monomers and dimers show markedly different patterns of activity response to ionic strength at given pH values: the reactions catalyzed by monomeric enzymes are highly modulated by salt, whereas those catalyzed by dimeric enzymes are not. In particular, at the reaction optimum the monomeric form of an enzyme is significantly more active than the dimeric one. 2. The optimum of the reaction catalyzed by a dimeric enzyme is shifted to higher ionic strengths in comparison with that of the reaction catalyzed by a monomeric enzyme. A model is proposed that could explain these results on the basis of the influence of ionic strength on the intramolecular dynamics of the enzyme molecule and its non-specific interactions with polyelectrolytic substrates.

Nucleic acid-protein interactions. Degradation of double-stranded RNA by glycosylated ribonucleases

1. Extensively glycosylated ribonucleases, like the enzymes from pig and horse pancreas, show a much higher activity on double-stranded RNAs than similarly charged, carbohydrate-free RNAases under stranded assay conditions (relatively high salt concentrations). Glycosylated pig and horse pancreas RNAases also show a larger destabilizing effect on double-stranded poly[d(A-T)] X poly[d(A-T)], than that displayed by bovine RNAase A under these conditions. Both activities show a similar dependence on the ionic strength of the medium. 2. A partial enzymic removal of the heterosaccharide side chains from pig and horse RNAases reduces but their degradative activity on double-stranded RNA and their destabilizing action on poly[d(A-T)] X poly[d(A-T)]. 3. These results are tentatively correlated with a modification of the microenvironment of the enzyme protein caused by its extensive glycosylation.

Ionic control of enzymic degradation of double-stranded RNA

The pattern of the degradation of various double-stranded polyribonucleotides by several ribonucleases (bovine RNAase A and its cross-linked dimer, bovine seminal RNAase, and pike-whale pancreatic RNAase) has been studied as a function of ionic strength and pH. It appears that (1) there is no direct correlation between the secondary structure of double-stranded RNA and its resistance against enzymatic breakdown, i.e., the stability of the secondary structure of double-helical RNA is not the main variable in the process. (2) The acstivity responses of the enzymes examined to changes of ionic strength and pH suggest that enzymic degradation of double-stranded RNA is mainly controlled by ion concentration, and that the process may fall within the phenomena interpreted by the theory of the ionic control of biochemical reactions advanced by Douzou and Maurel (Douzou, P. and Maurel, P. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 1013--1015). (3) The activity curves of the enzyme studied show, at a given pH, a shift toward higher ionic strengths as a function of the basicity of the enzyme protein. This finding explains the already observed correlation between number and/or density of positive charges of a ribonuclease molecule and its ability to attack double-stranded RNA in 0.15 M sodium chloride/0.015 M sodium citrate (SSC). (4) A careful analysis of the influence of ionic strength and pH on the reaction appears to be necessary in order to characterize a ribonuclease which shows activity towards double-stranded RNAs, and to allow a meaningful comparison between different enzymes capable of attacking these substrates.

Properties of purified salivary ribonuclease, and salivary ribonuclease levels in children with cystic fibrosis and in heterozygous carriers

Alkaline acid- and thermo-stable ribonuclease was isolated from human saliva of healthy subjects, children with cystic fibrosis, and from parents of the latter. The enzyme was purified about 180-200 times, and its ionic requirements, and specificity towards various substrates, characterized and compared with those for human pancreatic ribonuclease. The level of acid- and thermo-stable ribonuclease activity was five times higher in terms of enzyme level per ml saliva, and three times higher in terms of salivary protein, for 62% of children with cystic fibrosis, and for 73% of carrier subjects, relative to co-nrol groups. Assay of the level of this ribonuclease activity appears to be a promising diagnostic tool for detection of heterozygote carriers.

Primary structure of pronghorn pancreatic ribonuclease: close relationship between giraffe and pronghorn

Pancreatic ribonuclease from pronghorn (Antilocapra americana) was isolated and its amino acid sequence was determined from a tryptic digest of the performic acid-oxidized protein. Peptides were positioned by homology with other ribonucleases. Only peptides that differed in amino acid composition from the corresponding peptides of ox or goat ribonucleases were sequenced. In a most parsimonius tree of pancreatic ribonucleases, pronghorn and giraffe were placed together and these two were placed with the bovids, leaving the deer as a taxon separate from the other ruminants. The amino acid replacements that determine this tree topology are three rarely occurring replacements shared by pronghorn and giraffe. Notwithstanding their close phylogenetic relationship, both ribonucleases differ strongly in extent of glycosidation, net charge and antigenic properties.

Effect of exercise on ribonuclease activity in rat skeletal muscle

Distribution of ribonuclease activity (measured at pH 7.6) in subcellular fractions of homogenates of rat skeletal muscle was investigated in sedentary animals and after 8 weeks running program. Training increased ribonuclease activity (expressed as units of enzyme per g of muscle protein). There was no increase in nuclear fraction, but in both cytoplasmic and mitochondrial fractions the RNA-ase activity increased 42% and 45% respectively.

How much is secondary structure responsible for resistance of double-stranded RNA to pancreatic ribonuclease A?

1. Double-stranded f2 sus11 or Qbeta RNAs, resistant to bovine pancreatic RNAase A in 0.15 M NaCl/0.015 M sodium citrate (SSC), are quickly and completely degraded at 10-fold lower ionic strength (0.1 X SSC) under otherwise similar conditions. At this ionic strength the secondary structure of double-stranded RNA is maintained, as judged by the following: (a) the unchanged resistance of double-stranded RNA and DNA, under similar low ionic strength conditions, to nuclease S1 from Aspergillus oryzae, in contrast with the sensitivity of the corresponding denatured nucleic acids to this enzyme, specific for single-stranded RNA and DNA; (b) the co-operative pattern of the thermal-transition profile of double-stranded RNA (with a Tm of 89 degrees C) in 0.1 X SSC. 2. Whereas in SSC bovine seminal RNAase (RNAase BS-1) and whale pancreatic RNAase show an activity on double-stranded RNA significantly higher than that of RNAase A, in 0.1 X SSC the activity of the latter enzyme on this substrate becomes distinctly higher than that of RNAase BS-1, and similar to that of whale RNAase. 3. From these results it is deduced that the secondary structure is probably not the only nor the most important variable in determining the susceptibility double-stranded RNA to ribonuclease. Other factors, such as the effect of ionic strength on the enzyme and/or the binding of enzyme to nucleic acids, may play an important role in the process of double-stranded RNA degradation by ribonucleases specific for single-stranded RNA.