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The equilibrium binding patterns of complementary oligonucleotides to the native and denatured conformers of yeast transfer RNA 3 Leu have been determined. The pattern of binding to the native conformer follows that observed previously with other tRNAs. The results indicate that the anticodon loop and 3′ terminus are free in solution, and that all stems of the cloverleaf appear intact, although the dihydrouracil and “extra arm” stems are sufficiently weak to be subject to competitive binding by the probe oligomers. The T ΨC loop is also inaccessible to oligomer binding, while the dihydrouracil loop shows a low level of binding suggestive of oligomer competition with existing RNA structure. By contrast, in the denatured conformer the dihydrouracil loop and stem show strong oligomer binding characteristics of random RNA segments, whereas the anticodon loop no longer binds complementary oligomers. Binding to other regions remains unchanged, suggesting that the three major cloverleaf stems are intact. These observations are used as a basis for consideration of models for the two conformers.

The specific activities of 15 aminoacyl-tRNA synthetases in Saccharomyces cerevisiae were measured after growth under a variety of conditions that produced a range of cell-doubling times. The specific activity of each synthetase increased as cell-doubling time decreased. Control experiments eliminate the possibility that these results are due to preferential recovery of synthetases, or to the presence of activators in the faster growing cultures or inhibitors in the slower growing ones. These observations run counter to the expectation that synthetases in bacteria and yeast are negatively regulated by free amino acids, or, more likely, by aminoacyl-tRNA. In fact, as the growth medium was enriched, generation times decreased, and synthetase and aminoacyl-tRNA levels increased. It is suggested that cytoplasmic aminoacyl-tRNA synthetases may be more or less coordinately controlled such that their response to growth follows the pattern observed for ribosome production and RNA synthesis. This suggests the possibility of coordinated response of genes for components of the protein synthetic apparatus.

In the present investigations the molecular unfolding of yeast tRNA(Gly) has been studied by a combination of nuclear magnetic resonance spectroscopy, melting techniques, and relaxation kinetics. From these studies the following pathway of unfolding was found. In a coupled melting transition the tertiary, the DHU, and the anticodon structure are disrupted. This is followed by the melting of the acceptor arm, while the T psi C arm, which only contains G-C pairs, melts out last. Interestingly, during the first melting transition a new structure not belonging to the original cloverleaf structure is formed. The thermodynamic and kinetic parameters of the melting transitions were determined and are discussed in relation to earlier work. The present nuclear magnetic resonance (NMR) experiments as well as earlier studies show that the ring current calculations based on the cloverleaf structure provide a good first-order interpretation of the NMR spectra of tRNA.

A polynucleotide helical structure containing two strands of poly(A) and one of poly(U) is reported. As shown by spectroscopic observations, the complex only forms when the poly(A) strands are of Mr between 9000 and 50,000 (degree of polymerization congruent to 28-150), whereas the size of the poly(U) strand has no effect. This limitation may explain why poly(A.A.U) was not seen in previous investigations. The potential of the poly(A) tails of mRNA for formation of this triple helix and of A.A.U or/and A.A.T triplet formation to contribute to the binding of specific RNA strands to gene-encoding nucleic acid double helices are noted.

Characteristic fluorescence excitation and emission is induced by either acetone-sensitized 313 nm irradiation of mixtures of 8-bromoadenosine and adenosine or 254 nm irradiation of oligo- and polynucleotides containing adenine neighbors. The acetone-sensitized reaction involves cleavage of bromine from 8-bromoadenosine with activation of C-8, leading to formation of an 8,8-adenosine dehydrodimer. Comparable fluorescence properties arise in the unsensitized photoreaction of dApdA, pdApdA, ApA, poly(dA), poly(A), poly(dA.dT), and poly(dA.U). The previously unidentified adenine ultraviolet photoproduct described by Porschke has been isolated as several variants from solutions of pdApdA and poly(dA) irradiated at 254 nm. Based upon fluorescence spectra and mass spectra, these variants are shown to contain the 8,8-adenine dehydrodimer moiety.

High-resolution X-ray Diffraction by Single Crystals of Mixtures of Transfer Ribonucleic Acids

Ultraviolet photohydration and photodimerization reactions have been used to characterize the local environment of non-complementary uridine residues that are either isolated or clustered within the copolymer strand of poly(A,U) · poly(U) double helices. The initial photoreaction kinetics, measured from absorbance changes, are independent of the wavelength of measurement, and contain a fast-reacting component, absent in the analogous perfect helix, which corresponds quantitatively to the non-complementary residues. These results are consistent with an unstacked conformation of all non-complementary residues in these complexes. The initial kinetics demonstrate further that the bases are extrahelical and fully accessible to solvent, since their rate of photohydration is the same as for UMP. Photodimerization of these extrahelical U residues does not appear to occur within clusters of less than five residues, suggesting that neighboring extrahelical bases within smaller loops are stereochemically constrained from cyclobutane bond formation. Modification of extrahelical residues does not significantly disrupt adjacent helical secondary structure. Hence, the stoichiometric proportions of U residues that are intrahelical and extrahelical could be confirmed from the relation of hypochromicity to percentage photomodification. The resistance to photomodification of helical segments within the homopolymer-copolymer complexes (in sufficiently high counterion) is equivalent to that of the perfect analogue; the local environment of helical U residues within helices-with-loops and the perfect helix must, therefore, be essentially the same.

The 3′-OH terminus of bacteriophage MS2 RNA was selectively labelled with 3H. This was achieved by oxidation of the free 2′, 3′-diol group with sodium periodate to a dialdehyde, and reduction of the latter with tritiated sodium borohydride. Treatment of this RNA with alkali and separation of the hydrolysis products confirmed that only the terminal adenosine residue had been labelled. For the determination of the 3′-OH terminal sequence, RNA randomly labelled with 32P was admixed with terminally tritiated RNA, or, alternatively, the 32P-labelled RNA itself was terminally tritiated. Hydrolysis with ribonuclease T1 resulted in a mixture of oligonucleotides terminating in guanylic acid and a decanucleoside nonaphosphate containing the 3H label. This terminal oligonucleotide was isolated by ion-exchange chromatography on DEAE-Sephadex and DEAE-cellulose columns at different pH values and in the presence of 7 m-urea. The 3H label served to localize the terminal sequence and to estimate the purity. Alkaline hydrolysis of the isolated product indicated the composition (2 Ap, 2 Up, 5 Cp)A while pancreatic ribonuclease hydrolysis showed that each adenylic acid residue was followed by cytidylic acid. The complete sequence was established by partial hydrolysis with snake venom phosphodiesterase. The stepwise degradation products so obtained were labelled with 3H in the terminal nucleoside by the same procedure used for the entire RNA molecule, and then separated according to chain length. In this way, the nucleotide composition of each product could be determined on the basis of 32P activity, while its terminal nucleoside was determined on the basis of 3H activity. Both sets of data confirmed each other. The results, together with the known specificity of the ribonuclease T1, which had released the sequence, establish that MS2 RNA ends in …GpUpUpApCpCpApCpCpCpA. It is suggested that the termination signal for the translation into polypeptides is located at the left of this sequence and that the latter may be important for the initiation of replication.

The kinetics of interaction of equimolar mixtures of poly A and poly U in low and moderate [Na+] at several temperatures has been followed at three wavelengths: 259·0 mμ, where the hypochromic change per U residue incorporated into helix is the same for poly (A + U) and poly (A + U + U); 280·0 mμ, where the hypochromic change is due only to the formation of poly (A + U + U); and 283·5 mμ. where the hyperchromic change is due only to the formation of poly (A + U). The results indicate that, under conditions that do not allow formation of poly (A + U + U) (0·01 M -Na+ at 25°C), poly (A + U) is formed directly and at a rate that can be described by a standard second-order kinetic expression. By contrast, under conditions that do allow poly (A + U + U) formation (0·01 M -Na+ at M -Na+ at 25°C), the complete formation of poly (A + U) does not proceed directly. In addition to rapid formation of poly (A + U), substantial amounts of poly (A + U + U) appear with an induction period. This formation of incidental poly (A + U + U) reaches a maximum shortly before all the poly U is consumed. Thereafter, the formation of poly (A + U) is notably slower and proceeds at a rate that is indistinguishable from the rate of disappearance of a half-equivalent of poly (A + U + U). Complete formation in 0·2 M -Na+ at 25°C is not approached until approximately 72 hours. Four rate expressions derived from standard kinetic laws usually applied to reactions between small molecules are proposed as a quantitative description of the entire sequence of events.

Leucyl-tRNA synthetase from bakers' yeast was purified to homogeneity and characterized. The native enzyme has a molecular weight of 120,000 and contains two identical subunits that upon dissociation display very low aminoacylation activity. From the molecular weight and the sedimentation coefficient of the native enzyme (s20,w0 = 6.05 x 10-13) a value of 5.18 x 10-7 was calculated for d20,w0, which leads to a Stokes radius of 40.7 A. For a spherical molecule this radius corresponds to a volume of 282,000 A3. The kinetic parameters for the enzyme with its substrates are typical for this class of enzymes.

The kinetics of ultraviolet-induced photohydrate and photodimer formation in polyuridylate, and of photohydrate and deaminated photoproduct formation in polycytidylate have been determined by spectroscopic methods under a variety of solvent conditions. As the formation of photoproducts is sensitive to the local environment of the reacting residue, subtle inferences can be made concerning the secondary structure of these polynucleotides. For both 3′ uridylate and polyuridylate at 23°C, the kinetics of photohydration are single-hit over the sodium ion concentration range 0.1 to 1.0 m . The rates of photohydration of uridine, 3′ uridylate, and polymer are very similar, though only the nucleotide and polymer exhibit a progressive small depression in rate with increasing ionic strength. This dependence on ionic strength is due to a small kinetic salt effect on the photoreaction, and not to a counterion-dependent conformational effect. A small increase in the cross-section for photodimerization of polyuridylate accompanies the depression in photohydration rate. Denaturing organic solvents do not, however, diminish the yield of dimer. The results are consistent with polyuridylate being an extended randomly coiled polyelectrolyte at room temperature devoid of significant base-base interactions. By contrast with polyuridylate, the course of photohydrate formation in polycytidylate at 5°C follows two single-hit processes, the latter very similar in rate to that for cytidylate. Above 0.1 m -Na + , the initial kinetics are ionic strength independent. The results are consistent with the secondary structure of polycytidylate being a single-stranded helix of dynamically stacked residues.

Stoichiometry and thermodynamic properties of polyadenylate–polyuridylate helices containing varying proportions of near-randomly distributed non-complementary uridine residues were charactrized from an analysis of their mixing curves and melting profiles measured at 259 nm and at appropriate longer wavelength isochromic points. The noncomplementary residues in this homopolymer–copolymer system (in which the homopolymer has the capacity to readjust with respect to the residues with which it is in opposition) show absolute preference for an extrahelical conformation even when situated in … AAUAA … sequences and must occur therefore as single loops. As the frequency of extrahelical residues in creases, the electrostatic energy of these complexes becomes greater, and is particularly severe for the three-stranded helices. Thus, an adenyl-ate-uridylate copolymer containing 35.2 mole percent uridine residues does not form a three-stranded complex with polyuridylate even in 0.7M Na+at O°C. The imperfections introduced into the helix lattice by extrahelical residues decrease the cooperativity of thermal denaturation as well as Tm. However, for the helices with extrahelical residues in low frequency (∼1 per helix turn) only small increases in concentration of charge-neutralizing ions are required to bring Tm to the level of their perfect analogs. Two-stranded helices with a higher density of extra helical residues (∼5 per helix turn) show [Na+] dependence of Tm characteristic of perfect three-stranded helices. These findings together with the absence of an effect of these imperfections on the hypochromicity per base-pair suggest only minimal disruption of helix continuity or distortion of stacking interactions that normally in volve the base pairs or triplets.

Optical, hydrodynamic, and titration properties of polyribouridylic acid (poly U) were investigated in 0.2M Na+, pH 6. Above 15°C., poly U was shown to be a randomly coiled single chain, devoid of secondary structure. There is no shear rate dependence of viscosity; molecular weight is independent of ionic strength; there is virtually no conformationally dependent hypochromicity; no anomalous optical rotatory dispersion; and no anomalous titration behavior. The dependence of the sedimentation constant and intrinsic viscosity on molecular weight was determined. Poly U was shown to be the most highly expanded of all synthetic and naturally occurring randomly coiled polynucleotides.

Tritium exchange was used as a probe of transfer RNA structure in experiments with unfractionated tRNA (tRNA Unfrac and homogeneous tRNA 3 Leu from bakers' yeast. Exchange kinetics were measured over a range of ionic conditions that vary in ability to stabilize the secondary and tertiary structure of tRNA. The native conformations of both samples show the same kinetics of exchange. The kinetics for tRNA 3 Leu trapped in a denatured state in a “native” solvent are much faster, reflecting the conformation and not the ionic medium. In 0.1 M-Na + , where tRNA 3 Leu is denatured, the kinetics for tRNA Unfrac are intermediate between those for native and denatured tRNA 3 Leu , suggesting that in this solvent at 0 °C some tRNAs are denatured whereas other are still native. Upon further lowering of Na + concentration, tRNA Unfrac shows increasingly faster exchange, suggesting complete electrostatic denaturation of the tertiary structure of all the tRNAs in the sample, and even disruption of secondary structure. Extrapolation of the essentially linear early-time kinetics to zero time provides minimal estimates of the number of slowly exchanging hydrogens. For native tRNA 3 Leu the number is 111±2 hydrogens, whereas for the trapped denatured conformation it is only 95±2. This difference reflects a smaller number of hydrogen-bonded bases in the denatured conformation. In 1 M-Na + , 101±2 slowly exchanging hydrogens are found for the native tRNA 3 Leu conformation, suggesting an incompletely formed native structure. For native tRNA Unfrac the comparable number is 101±3. These numbers of slowly exchanging hydrogens in the native conformations are consistent with tertiary structural hydrogen-bonding. Furthermore, this tertiary structure must be responsible for the slower exchange by native tRNA. The observed numbers of exchangeable hydrogens provide a basis for comparison of hydrogen-bonding interactions in native and denatured tRNA conformations. The mechanism of renaturation was also investigated, using tritium exchange as a monitor of perturbation of base pairing during the transition. When tRNA Unfrac in low Na + is renatured by addition of Mg 2+ during tritium exchangeout, a burst of exchange or “spillage” of tritium is detected. This suggests that a fraction of the base pairs of the rapidly renaturing tRNAs in the mixture is disrupted during renaturation. In that event, and by analogy with tRNA 3 Leu , part of the base-pairing arrangement of the denatured conformations may not be preserved in the native state; and if the native conformation includes the full “cloverleaf” pattern of secondary structure, that pattern may not be intact in some denatured conformations.

tRNA Leu -Synthetase from baker's yeast has been crystallized from ammonium sulfate solution. The crystals are of orthorhombic symmetry, falling into space group [unk] or [unk] with a = 75.5 Å, b = 110.7 Å, and c = 124.0 Å. This unit cell contains four molecules of the native (dimeric) enzyme, with eight asymmetric units, each corresponding to the enzyme subunit. Diffraction patterns obtained for the three major projections contain reflections to spacings of less than 3.5 Å, indicating that this crystal form is amenable to structure analysis to atomic resolution.

The often employed practice of reporting continuous variation experiments with polynucleotides in terms of relative ultraviolet absorbance, to better portray the hypochromicity of the complex whose stoichiometry is under investigation, is shown to be incorrect. Ways in which this type of plot can lead to erroneous conclusions are discussed. It is concluded that plotting the observed absorbance measurements is the appropriate method for data analysis and presentation.

There is a multiplicity of etiological factors in human cancer; psychic factors are probably neither the most important nor the most constant. Nonetheless, owing to the potential infinity of psychological variables, it is necessary to formulate hypotheses in an operational manner. The casuistic method allows a thorough clinical study of objective events, of the manner in which the patient lives them and, especially, of what they mean to him. Such an approach, however, has shortcomings which must be accepted: it cannot administer proof of the hypotheses it advances. The superiority of prospective methods over retrospective methods seems indisputable for there is no evidence that personality traits are not modified by the fatal threat of a serious illness. This criticism also holds for the modified retrospective methods, the so-called predictive methods. Research focussed on particular cancers with a study of limited psychological variables seems desirable.

A 62-year-old white male suffering from plasmacytosis with monoclonal gammopathy developed gastrointestinal bleeding, microscopic hematuria, proteinuria and progressive azotemia. A renal biopsy demonstrated capillary wall thickening, mesangial hyperplasia, crescent formation and subepithelial humps. Serum immunoglobulin analysis showed IgG of lambda type. Serum complement, ANA, ASO titer and cryoglobulin were normal. It is suggested that proliferative glomerulonephritis with subepithelial humps should be added to forms of renal pathology that can be seen in plasma cell dyscrasias.

The binding of a variety of deoxyribo and ribo homo- and copolynucleotide complementary duplexes to agarose-linked homopolynucleotide affinity columns has been studied. The results provide information concerning the specificity of recognition of complementary base pairs of nucleic acids through a mechanism that involves triple-helix formation under physiological conditions of ionic strength, pH, and temperature. The method employed made it possible, for the first time, to survey the full range of base triplets conceivable from the canonical nucleic acid bases and, in addition, hypoxanthine and thereby to differentiate between those triplets which can and cannot form. Certain previously observed features of the stereochemistry of double-helical targets for third-strand binding are confirmed, and some unrecognized features are elaborated. These include a general requirement for clusters of purine residues in one strand, protonation of third-strand C residues, the ability of natural third-strand residues to distinguish between A.T/U and G.C base pairs, and a capacity of third-strand (unnatural) I residues to recognize all base pairs within such clusters. Thus, the basis for a third-strand binding code is demonstrated.

Crystals of pairs of H-bonded nucleic acid bases are generally grown from nonaqueous solutions. We have been able to predict the H-bonded configuration of most of the base pairs in such crystals by using an empirical-potential function we recently developed for calculating the energetics of such interactions in chloroform solution. The following configurations were computationally predicted to predominate and are those observed in crystal structures: the Watson-Crick G.C configuration instead of two competing configurations; the Hoogsteen-type configurations for A.T, A.U, and A.br5U instead of Watson-Crick-type configurations; the Watson-Crick-type configurations for 2-aminopurine.br5U instead of the purine N3-type configuration; the Watson-Crick-type configurations for 8-bromo-2,6-diaminopurine.T instead of the Hoogsteen or purine-N3-type configurations; the syn-anti configuration for br8A.br8I instead of the anti-anti configuration; the Watson-Crick-type configurations for br8A.br5U instead of the Hoogsteen-type configurations; and the Hoogsteen-type configurations for me8A.T instead of the Watson-Crick configurations. In addition, the H-bonded base triplet br5U.2,6-diaminopurine.br5U was calculated to have Hoogsteen and Watson-Crick-type configurations but not the purine N3-type configuration. Apparently, lattice forces and chance nucleation of a minor base pairing configuration are not significant when the stability difference between the preferred and alternative configurations exceeds a relatively small value. In one case, in order to correctly predict the base pairing configuration in the crystal, it was necessary to include a contribution due to a C--H...O bond, suggesting that this type of H bond can make a significant contribution to base pair stability.

The renaturation of inactive yeast tRNA Leu by the calf thymus unwinding protein “UP 1” is shown to proceed through formation of a protein-RNA complex, which, in the presence of Mg, dissociates with concommitant renaturation of the RNA. The inhibition of this renaturation by single stranded poly- and oligonucleotides indicates that the action of UP 1 is a consequence of its affinity for single-stranded segments of the tRNA, with an optimal length of ∼9 nucleotides. UP 1 is not found in calf thymus nuclei isolated in sucrose solution; and the cytoplasmic supernatant contains UP 1 in an amount comparable to that found in whole thymus cells. Crude calf thymus chromatin contains a protein that has the same mobility on SDS-polyacrylamide gels as UP 1 (19, 000 daltons), but greater mobility on acetic acid-urea gels. This protein has some effectiveness in renaturing tRNA, but differs from UP 1 in several other respects.

Publisher Summary This chapter describes that secondary structure in nucleic acids generally consists of double-stranded helical regions that are stabilized by a variety of interbase forces. Bases in the opposing strands are regularly oriented with respect to the long axis of the helix by at least two hydrogen bonds, and only certain oppositions seem to be allowed, usually a purine and a pyrimidine. In the chapter, the variety of codon-anticodon interactions observed in protein synthesis suggests that there can be some flexibility in these requirements, such that pairing need not be restricted to base oppositions that are precisely accommodated by the one-dimensional lattice of a long double helix. The conformational possibilities of base oppositions that fall outside these complementary categories also merit definition, particularly in view of several recent indications that noncomplementary residues may have important functional roles in naturally occurring nucleic acids. The conformational alternatives available to noncomplementary residues within helical regions logically fall into two general categories, one being intrahelical and the other extrahelical or “looped-out’’. The conformational class that is adopted in a particular case depends upon the interplay of some factors, such as: (1) the degree of local stereochemical distortion of the helix when the non-complementary residue is left intrahelical, (2) conformatianal entropy, (3) solvation, (4) charge effects, and (5) the relative stacking tendency of the noncomplementary base

An acoustic stress increases the metastatic spread of polyoma virus-induced tumors in syrian Hamsters. To try to elucidate the mechanism of this action, we studied the effect of stress on the protection afforded by the virus against tumour challenge. Exposure to ultrasonics during the immunization period markedly decreased the protective effect of the virus.

Complexes of inert (slowly exchanging) multivalent transition metal cations and neutral ligands were found to effectively replace Mg 2+ as stabilizers of the biologically active “native” form of yeast tRNA Leu 3 . From an analysis of the sigmoidal dependence of the denatured native conformational equilibrium on complex cation concentration at 25°C in 0·15 m -KCl at pH 7·4, it is calculated that about five complex cations must be tightly bound to stabilize the native conformer. Since this number is larger than the three binding sites previously observed for alkaline earth metal ions and polyamines, there must be sites of high negative charge on the tRNA accessible to Mg 2+ and spermidine 3+ but not to the more bulky complex cations. In that case, the complex cations must occupy additional sites in order to stabilize the native conformer. The relative effectiveness with which the cations stabilize the native conformation is: Co(NH 3 ) 6 3+ ≳ Pt(en) 2 (en-H) 3+ > Co(en) 3 3+ > Ir(NH 3 ) 5 Cl 2+ ⋍ Pt(NH 3 ) 4 2+ . This order is consistent with general electrostatic considerations. Hydrogen bonding of the co-ordinated amines to tRNA phosphates could enhance the affinity of Co(NH 3 ) 6 3+ . Since the complex cation-tRNA association constants are even larger than those of the cobalt complexes with multivalent inorganic anions, a cluster of at least three phosphates could constitute the binding site of each tightly bound cation in tRNA.

Renal atherosclerosis and fibromuscular dysplasia are the most common causes of curable human renovascular hypertension and renal failure. Vascular reconstruction often preserves renal function, but renal failure is rarely reversed, especially after days of anuria. We report a case of a 23-year-old woman who as a child underwent a nephrectomy for congenital hydroureter and renal hypoplasia. She later experienced fibromuscular dysplasia of the remaining renal artery, which ultimately progressed to a complete occlusion and 31 days of total anuria. The patient was revascularized, and within 2 months renal function returned with a blood urea nitrogen and creatinine of 9.0 and 1.0 mg/dl, respectively. After a follow-up of 6 months the patient's blood pressure remained 120/80 to 130/80 mm Hg without administration of hypertension medication. In this report we emphasize that under selected circumstances a kidney can survive prolonged ischemia and that delayed revascularization may reestablish renal function.

One lung of each dog was exposed to smoke from burning pine wood, while the other was subjected to acute hypoxia. Angiotensin-1-converting enzyme (ACE) activity in biopsied tissue of the smoke-exposed lung was markedly increased immediately after the injury and even higher 30 minutes later. No change in ACE activity was observed in the hypoxic contralateral lung. Serum ACE activity did not change significantly following anesthesia and before smoke inhalation. Serum aldosterone and cortisol levels increased at this juncture. Smoke inhalation caused intra-alveolar hemorrhages and edema. Pulmonary and systolic, diastolic and mean pressures, pulmonary capillary, wedge pressure, cardiac output and systemic and pulmonary arteriolar resistances remained unchanged throughout the experiment. The changes of ACE activity are presumably a direct effect of smoke inhalation. They are seen as an early response of the lung endothelial cells to many types of injury (chronic hypoxia, bleomycin or monocrotaline administration) and may represent an important step in the development of the organ's response to the injury.

Using ultraviolet spectral techniques, the hydrogen bonding schemes have been determined for the A·C and I·U (as analogue of G·T) base mispairs in heli-ces dominated by the Watson-Crick pairs A·U or I·C. Each mispair contains one of its bases in the unfavored tauto-meric form (enol or imino), while the other base is in the favored tautomeric form (keto or amino). This per-mits mispairs with hydrogen bonding schemes that are fully compatible with the stereochemical constraints of a Watson-Crick helix. Which of the bases in a mispair assumes the unfavored tautomeric form depends upon the resultant stacking interactions of the bases of the mis-pair with their nearest neighbor Watson-Crick pairs. Thus, when the A-C mispair is surrounded by A·U nearest neighbors in the pair sequence A·U, A·C, A·U, the un-favored tautomeric form is assumed by the C residue; and when surrounded by I·C in the pair sequence I·C, A·C, I·C, it is the A residue that assumes the unfavored tautomeric form. The unfavored tautomer in an I·U mispair also changes from U enol to I enol as the nearest neighbors are varied from I·C to A·U. Such changes in the tautomeric components of the mispairs can be rational-ized by recognizing that the favored and unfavored tauto-mers of each of the bases have different tendencies to stack with particular nearest neighbors. Apparently, favorable stacking interactions provide sufficient ener-gy to overcome the cost of stabilizing an unfavored tautomer in a base pair. These findings indicate that the A·C and G·T pairs have the energetic and stereochem-ical characteristics necessary for them to serve as the mispairs leading to transition mutations during DNA rep-lication; and sequence, by way of local stacking inter-actions, is seen to have an important role in determining a differential in mutation frequencies at different sites in a genome.

An invasive paraganglioma of the posterior mediastinum caused spinal cord compression in a 31 year old women. Electron microscopic examination of the paraganglioma invading the epidural space revealed numerous dense-cored granules in the cytoplasm of the tumour cells. We are reporting this case to present the ultrastructure of mediastinal paraganglioma, and to call attention to an unusual cause of spinal cord compression.