Showing papers by "University of Würzburg published in 1988"

Increased iron (III) and total iron content in post mortem substantia nigra of parkinsonian brain.

Abstract: Significant differences in the content of iron (III) and total iron were found in post mortem substantia nigra of Parkinson's disease There was an increase of 176% in the levels of total iron and 255% of iron (III) in the substantia nigra of the parkinsonian patients compared to age matched controls In the cortex (Brodmann area 21), hippocampus, putamen, and globus pallidus there was no significant difference in the levels of iron (III) and total iron Thus the changes in total iron, iron (III) and the iron (II)/iron (III) ratio in the parkinsonian substantia nigra are likely to be involved in the pathophysiology and treatment of this disorder

Time course of mechanosensitivity changes in articular afferents during a developing experimental arthritis.

Abstract: 1. In 37 cats anesthetized with alpha-chloralose recordings were made from single-afferent units of the medial articular nerve (MAN) of the right knee joint. First the mechanosensitivity of such units was characterized while the joint was in normal condition. Thereafter, keeping the afferents under continuous observation, an experimental arthritis was induced by injecting kaolin and carrageenan into the joint cavity. The effects of the developing arthritis including the time course of the changes were studied on low- and high-threshold units and on afferents that had no mechanosensitivity in the normal joint. 2. The arthritis increased the mechanosensitivity in the majority of the low-threshold units, i.e., in units that responded already in the normal joint to movements in the working range. Enhanced responses to movements were found for 12 of 16 thick myelinated group II, 10 of 10 fine myelinated group III, and 1 of 3 unmyelinated group IV afferents. The augmentation of reactions developed in most cases within the first hour after the injection of the inflammatory compounds, sometimes starting immediately after the injection. A further rise of the mechanosensitivity was observed within the following 2-4 h. In most group III units enhanced responses for movements were accompanied by an induction or increase of resting discharges. In 1 group II and 1 group IV unit spontaneous activity developed in the absence of any change of movement-sensitivity. 3. The inflammation led to enhanced mechanosensitivity in high-threshold afferents, i.e., in units that responded in the normal joint only to noxious movements exceeding the working range of the knee. One group II, 10 of 12 group III, and 5 of 10 group IV units of this type became responsive to movements in the working range during development of arthritis, in most cases within the second to third hour after induction of inflammation with a further increase later on. In a high proportion of these units resting activity was induced too. Few high-threshold units developed spontaneous discharges but no responses to movements in the working range. The time course for development of resting activity was similar to that for lowering of the mechanical threshold. 4. The experimental arthritis induced afferent activity in 1 of 2 group III and 10 of 14 group IV units that in the normal joint were unresponsive to local mechanical stimulation and to innocuous/noxious movements (but responsive to a bolus of a KCl-solution applied intraarterially close to the joint).(ABSTRACT TRUNCATED AT 400 WORDS)

Zeaxanthin and the Heat Dissipation of Excess Light Energy in Nerium oleander Exposed to a Combination of High Light and Water Stress.

Abstract: Upon termination of watering of plants of Nerium oleander exposed to high light, photochemical efficiency became reduced as leaf water content decreased. Evidence is presented that this type of photoinhibition reflects to a substantial degree radiationless dissipation of excitation energy, probably mediated by the carotenoid zeaxanthin. During the imposition of water stress, the zeaxanthin content of leaves increased at the expense of violaxanthin and β-carotene as a water deficit developed over a period of several days. The increase in zeaxanthin content was linearly related to an increase in the rate of radiationless energy dissipation in the antenna chlorophyll as calculated from the characteristics of chlorophyll a fluorescence measured with a pulse amplitude modulated fluorometer at room temperature. The increase in the rate of radiationless dissipation was also linearly related to a decrease in PSII photochemical efficiency as indicated by the ratio of variable to maximum fluorescence. Leaves of well-watered shade plants of N. oleander exposed to strong light showed a similar increase in zeaxanthin content as sun leaves of the same species subjected to drought in strong light. Shade leaves possessed the same capacity as sun leaves to form zeaxanthin at the expense of both violaxanthin and β-carotene. The resistance of this species to the destructive effects of excess light appears to be related to interconversions between β-carotene and the three carotenoids of the xanthophyll cycle.

Measuring P700 Absorbance Changes around 830 nm with a New Type of Pulse Modulation System

Abstract: Abstract A new measuring system for monitoring absorbance changes around 830 n m is described, which was developed by modification of a commercially available pulse modulation fluorometer. All modifications concern the emitter-detector unit of the fluorometer, such that only this unit needs to be exchanged when changing from fluorescence to absorbance measurements and vice versa. The new system is shown to be well-suited for measuring redox changes of P700, the reaction center of photosystem I, in intact leaves and isolated chloroplasts. The observed kinetic changes at 830 nm in response to single turnover or multiple turnover saturating flashes are practically identical to those previously measured around 700 nm . The signal/noise ratio is sufficiently high to give well-resolved kinetics without signal averaging. W h e n P700 is oxidized by far-red background light, valuable information on the state of the intersystem electron transport chain is given by the re-reduction kinetics induced by single or multiple turnover saturating flashes. Such measurements are facilitated by the use of poly-furcated fiberoptics. With intact leaves, almost identical responses are found when measuring through the leaf (transmission mode) or from the leaf surface (remission mode). Modulated chlorophyll fluorescence can be measured in parallel; application of saturation pulses for fluorescence quenching analysis produces transient P700 oxidation without oversaturating the measuring system. The information on the P700 redox state complements that obtained from fluorescence measurements, yielding a new practical tool in plant physiological research.

Excitation and sensitization of fine articular afferents from cat's knee joint by prostaglandin E2.

Abstract: 1. In cats anaesthetized with alpha-chloralose extracellular recordings were made from fine afferent units belonging to the medial articular nerve of the knee joint. The excitatory and sensitizing effects on articular afferents of prostaglandin E2 (PGE2) applied intra-arterially close to the joint were examined. 2. Bolus injections of PGE2 doses of 0.03-30 micrograms excited about 60% of both the group III (conduction velocity 2.5-20 m/s) and the group IV units (conduction velocity less than 2.5 m/s). The duration and size of the responses were dose dependent consisting in most cases of low-frequency discharges which lasted up to several minutes. Excitation was found among afferents with low and high mechanosensitivity. 3. Among the group III units PGE2 sensitized 64% for their responses to movements and 50% for their responses to bradykinin (applied intra-arterially close to the joint). Sensitization did not depend on the mechanical threshold previous to chemical stimulation. Among the group IV units PGE2 sensitized only 25% for their responses to movements but 75% for their reactions to bradykinin. In group IV fibres a low mechanical threshold predisposed for sensitization to movements and a higher threshold for sensitization to bradykinin. 4. Some units were sensitized and excited, others were either sensitized or excited and some units were not affected by PGE2. We conclude that PGE2 induces in a large proportion of fine articular afferents of normal joints discharges which are similar to those induced by an experimental inflammation. Thus PGE2 may be an inflammatory mediator which has a major role in the generation of the afferent activity developing in the course of an arthritis.

Biosynthesis and biotransformation of glutathione S-conjugates to toxic metabolites.

Abstract: The material presented in this review deals with the hypothesis that the nephrotoxicity of certain halogenated alkanes and alkenes is associated with hepatic biosynthesis of glutathione S-conjugates, which are further metabolized to the corresponding cysteine S-conjugates. Some glutathione or cysteine S-conjugates may be direct-acting nephrotoxins, but most cysteine S-conjugates require bioactivation by renal, pyridoxal phosphate-dependent enzymes, such as cysteine conjugate beta-lyase (beta-lyase). The biosynthesis of glutathione S-conjugates is catalyzed by both the cytosolic and the microsomal glutathione S-transferases, although the latter enzyme is a better catalyst for the reaction of haloalkenes with glutathione. When glutathione S-conjugate formation yields sulfur mustards, as occurs with vicinal-dihaloethanes, the S-conjugates are direct-acting toxins. In contrast, the S-conjugates formed from fluoro- and chloroalkenes yield S-alkyl- or S-vinyl glutathione conjugates, respectively, which are metabolized to the corresponding cysteine S-conjugates by gamma-glutamyltransferase and dipeptidases; inhibition of these enzymes blocks the toxicity of the glutathione S-conjugates. The cysteine S-conjugates must be metabolized by beta-lyase for the expression of toxicity; the beta-lyase inhibitor aminooxyacetic acid blocks the toxicity of cysteine S-conjugates, and the corresponding alpha-methyl cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates. Homocysteine S-conjugates are also potent cyto- and nephrotoxins. The high renal content of gamma-glutamyltransferase and the renal anion transport system are probably determinants of kidney tissue as a target site. Biochemical studies indicate that renal mitochondrial dysfunction is produced by the cysteine S-conjugates. Finally, some of the glutathione and cysteine conjugates are mutagenic in the Ames test, and reactive intermediates formed by the action of beta-lyase may contribute to the nephrocarcinogenicity of certain chloroalkenes.

The repeat domain of Escherichia coli haemolysin (HlyA) is responsible for its Ca2+-dependent binding to erythrocytes.

Abstract: The haemolysin protein (HlyA) of Escherichia coli contains 11 tandemly repeated sequences consisting of 9 amino acids each between amino acids 739 and 849 of HlyA. We removed, by oligonucleotide-directed mutagenesis, different single repeats and combinations of several repeats. The resulting mutant proteins were perfectly stable in E. coli and were secreted with the same efficiency as the wild-type HlyA. HlyA proteins which had lost a single repeat only were still haemolytically active (in the presence of HlyC) but required elevated levels of Ca2+ for activity, as compared to the wild-type haemolysin. Removal of three or more repeats led to the complete loss of the haemolytic activity even in the presence of high Ca2+ concentrations. The mutant haemolysins were unable to compete with the wild-type haemolysin for binding to erythrocytes at low Ca2+ concentrations but could still generate ion-permeable channels in artificial lipid bilayer membranes formed of plant asolectin, even in the complete absence of Ca2+. These data indicate that the repeat domain of haemolysin is responsible for Ca2+-dependent binding of haemolysin to the erythrocyte membrane. A model for the possible functional role of Ca2+ in haemolysis is presented.

Hemolysin supports survival but not entry of the intracellular bacterium Listeria monocytogenes.

Abstract: The gram-positive bacterium Listeria monocytogenes is a facultative intracellular pathogen. The only known property of L. monocytogenes which has been shown to be involved in virulence is a hemolysin, listeriolysin (J. L. Gaillard, P. Berche, and P. Sansonetti, Infect. Immun. 52:50-55, 1986; S. Kathariou, P. Metz, H. Hof, and W. Goebel, J. Bacteriol. 169:1291-1297, 1987). Using our previously obtained transposon Tn916-induced hemolysin-negative mutants of L. monocytogenes Sv1/2a (Mackaness strain), we demonstrated that the loss of hemolysin reduced significantly the rate of survival of the bacteria in mouse peritoneal macrophages but did not reduce their uptake. It was further shown that virulent L. monocytogenes strains could invade the mouse embryo fibroblast 3T6 cell line, i.e., mammalian cells which are nonprofessional phagocytes. This uptake was inhibited by cytochalasin B and hence seems to be accomplished by parasite-induced endocytosis. Hemolysin was not essential for this step. Strains of other Listeria species could not efficiently penetrate the 3T6 cells.

Evolutionary diversity of reverse (R) fluorescent chromosome bands in vertebrates

Abstract: Mitotic chromosomes, interphase cell nuclei, and male meiosis of 41 species representing all vertebrate classes were analyzed with distamycin A/mithramycin counterstaining. The purpose of the study was to recognize differences and common characteristics in the reverse (R) fluorescent banding patterns in the chromosomes of vertebrate species at various stages of evolution. In contrast to the warm-blooded mammals and birds, the euchromatic segments in the chromosomes of most reptiles, amphibians, and fishes contain no multiple fluorescent R-bands. This is thought to be due to the absence of the long homogeneous regions (isochores) in the DNA of the cold-blooded vertebrates. Distamycin A/mithramycin banding specifically reveals the GC-rich constitutive heterochromatin in all vertebrates. In most of the vertebrate chromosomes examined, the heterochromatic regions have opposite staining properties with mithramycin and quinacrine. Mithramycin labels the nucleolus organizer regions very brightly in the karyotypes of fishes, amphibians, reptiles and birds, but not of mammals. The lack of mithramycin fluorescence at the nucleolus organizer regions of mammals is attributed to the relatively low level of redundancy of the GC-rich ribosomal DNA in their genomes. Studies on the various meiotic stages of the cold-blooded vertebrates show that the mithramycin labeling of the nucleolus organizers is independent of their state of activity. This can be confirmed by mithramycin fluorescence at the nucleoli of actinomycintreated cells.

Aerogels — highly tenuous solids with fascinating properties

Abstract: Aerogels are sol-gel derived, supercritically dried materials with porosities up to 98%. The pore structure extends from scale lengths of about 1 to 100 nm, showing micro- and macro-porosity as well. The solid thermal conductivity of aerogels is 2 to 3 orders of magnitude smaller than for massive silica glass. Infrared radiative heat transport in SiO2-aerogels is sufficiently suppressed at ambient temperatures. Translucent aerogels thus are suitable as evacuated superinsulating spacers for all kinds of window systems with thermal loss coefficients as low as 0.5 W/(m2 K) for thicknesses around 15 mm. Aerogels are exceptional acoustic material too, with a sound velocity in the order of 100 to 300 m/s, corresponding to an acoustic impedance of only 104 to 105 kg/(m2 s) and a Young's modulus in the range of 106 to 107 N/m2. Due to the large specific surface area of about 800 m2/g aerogels might be useful as catalytic substrates. Last but not least aerogels are considered extremely pure precursor materials for the production of glass fiber.

DNA rearrangements in organ-specific variants of polyomavirus JC strain GS.

Abstract: Variants of JC virus (JCV) strain GS were isolated directly from the central nervous system (variant GS/B) and the kidney (variant GS/K) of a patient with progressive multifocal leukoencephalopathy and were cloned and sequenced The genomes of the isolates were shown to be nearly identical in the nucleotide sequences of their protein-coding regions, suggesting that both had originated from a single infecting JCV genome In contrast, the arrangement of the putative elements of transcriptional control revealed considerable differences The tandemly repeated elements found twice within the enhancer region of JCV GS/B variant were not present in the GS/K variant The missing elements were replaced by DNA segments containing simian virus 40 and adenovirus E1A core enhancer elements These differences in the organ-specific GS variants suggest that rearrangements within elements of transcriptional control might be involved in altering the virus-cell interaction in the course of a JCV infection

Fanconi anemia mutation causes cellular susceptibility to ambient oxygen.

Abstract: The gene defect causing the Fanconi anemia (FA) phenotype appears to be expressed at the cellular level, since FA fibroblasts show a protracted course of explant outgrowth, a diminished in vitro life span, and very poor cloning. We show that exposure of FA fibroblasts to hypoxic (5% v/v oxygen) culture conditions restores their growth in vitro to near normal. Exposure to elevated oxygen tension (35% v/v) causes accumulations of FA cells in the S and G2/M phases of the cell cycle that are in significant excess of those seen in heterozygote and control strains. In the absence of evidence for defective cytoplasmatic radical scavenging systems, these observations suggest increased nuclear susceptibility to ambient oxygen as cause of the FA cellular phenotype.

Water status related photosynthesis and carbon isotope discrimination in species of the lichen genusPseudocyphellaria with green or blue-green photobionts and in photosymbiodemes.

Abstract: Green lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δ13C values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δ13C values in lichens can be explained by a C3 carboxylation system and the various effects of different limiting processes for photosynthetic CO2 fixation. If carboxylation is rate limiting, there will be a strong discrimination of13CO2, at high internal CO2 partial pressure. The resulting very low δ13C values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO2 becomes more and more rate limiting and the internal CO2 pressure decreases, the13C content of the photosynthates increases and less negative δ13C values results, as are found for blue-green lichens.

The projection of the medial and posterior articular nerves of the cat's knee to the spinal cord.

Abstract: We studied the spinal projections of the medial and posterior articular nerves (MAN and PAN) of the knee joint in the cat with the aid of the transganglionic transport of horseradish peroxidase. The afferent fibers of the MAN entered the spinal cord via the lumbar dorsal roots L5 and L6 and those of the PAN entered via the dorsal roots L6 and L7. Within the dorsal root ganglia, most labeled neurons had small to medium diameters. A relatively higher number of medium-size cell bodies were labeled from the PAN than from the MAN. In the spinal cord labeled MAN afferent fibers and terminations were most dense in the L5 and L6 segments, and those of the PAN were most dense in L6 and L7, that is, in the respective segments of entry. Labeled afferent fibers from both nerves projected rostrally at least as far as L1 and caudally as far as S2. Labeled fibers were found in Lissauer's tract as well as in the dorsal column immediately adjacent to the dorsal horn. In the spinal gray matter, both nerves had two main projection fields, one in the cap of the dorsal horn in lamina I, the other in the deep dorsal horn in laminae V-VI and the dorsal part of lamina VII. Both nerves, but particularly the PAN, projected to the medial portion of Clarke's column. No projection was found to laminae II, III, and IV of the dorsal horn or to the ventral horn. Since these findings parallel observations on hindlimb muscle afferent fibers, the present data support the existence of a common pattern for the central distribution of deep somatic afferent fibers.

Joint receptors and kinaesthesia.

Abstract: This is a brief review of the role of joint receptors in kinaesthesia, the sense of position and movement of the limbs. It is proposed that joint receptors are concerned with signalling joint movement but not . joint position, at least over most of a joint's working range. This view is supported by recent psychophysical experiments which also suggest that the sense of position derives from activity in muscles acting about a joint. There is continuing controversy over interpretation of animal experiments and whether joint receptors are able to signal joint position. One source of confusion has been the fact that some joint nerves contain afferent fibres arising from nearby muscles. It is concluded that on present evidence joint receptors are unlikely to play a major role in the sense of position, although they have important functions, signalling joint movement, acting as joint limit detectors and as nociceptors. The role of joint receptors in kinaesthesia is a subject which has been discussed many times, especially in recent years. Here is yet one more account, one which is not meant to be a comprehensive review of the topic, but which hopes to highlight some interesting aspects. The term kinaesthesia, despite its literal meaning, is now widely used for the sense of position as well as movement of body parts. It will be argued that these are two distinct sensations and the afferent signals that give rise to them may well take origin in different sets of receptors. The changing views of the role of joint receptors in kinaesthesia are well described in Matthews' 1982 review. Briefly, while in the 1950's and 1960's joint receptors were thought to be the principal receptors involved, the focus of attention shifted when evidence was obtained which implicated muscle receptors in kinaesthesia and when doubt was cast on the

Two different adenylyl cyclases in brain distinguished by monoclonal antibodies.

Abstract: Four monoclonal antibodies, raised against the 115-kDa adenylyl cyclase from bovine brain [Pfeuffer, E. et al. (1985) EMBO J. 4, 3675–3679] have been selected and designated BBC-1 to BBC-4. BBC-1 and BBC-3 are highly specific for the 115-kDa enzyme from bovine brain. The two other antibodies, BBC-2 and BBC-4, recognize an additional 150-kDa adenylyl cyclase in bovine brain, but also in brain tissue from other species. In membranes from lung and myocardium (bovine and rabbit) only the 150-kDa species is detected by the crossreacting antibodies BBC-2 and BBC-4.The two adenylyl cyclases from brain can be separated by calmodulin-Sepharose: only the enzyme of 115 kDa but not that of 150 kDa was retained by the affinity resin and could be stimulated by Ca2+/calmodulin. The data obtained with these antibodies of defined specificity provide for the first time direct evidence for the presence of two distinct adenylyl cyclase species in brain tissue.

Pathogenesis of myasthenia gravis. Acetylcholine receptor-related antigenic determinants in tumor-free thymuses and thymic epithelial tumors.

Abstract: The authors describe an immunohistologic study of acetylcholine receptor (AChR)-related antigenic determinants in tumor-free thymuses of myasthenia gravis (MG) patients (13 cases) and nonmyasthenic controls (10 cases) and in thymic epithelial tumors of patients with MG (8 cases) and without MG (6 cases). Monoclonal antibodies (MAbs) to the cytoplasmic part and to the extracellular main immunogenic region (MIR) of the alpha subunit of AChRs were used. Their intrathymic binding sites were defined by double-immunostaining, and compared with alpha-bungarotoxin (alpha-Bgt) labeling demonstrated by fluorescence microscopy. Tumor-free thymuses of MG patients and control patients contained cytoplasmic AChR epitopes and alpha-Bgt binding sites on myoid cells and some epithelial cells. Only myoid cells also expressed extracellular MIR epitopes, suggesting that they bear complete AChRs, and are important targets for the autoimmune attack in tumor-free MG thymus. Evidence that AChR-related antigenic determinants of epithelial cells are also significant for MG is provided by our findings in thymic epithelial tumors. All eight tumors with MG but only two out of six tumors without MG showed cytoplasmic AChR epitopes and alpha-Bgt binding sites on neoplastic epithelial cells. Myoid cells and MIR epitopes did not occur in the neoplasms, but in some tumor-free thymic remnants beside thymomas. It is assumed that nonneoplastic and neoplastic thymic epithelial cells contain only incomplete AChRs or AChR-like molecules. The different expression of AChR epitopes in thymic epithelial tumors and tumor-free thymuses might explain some of the heterogeneous region specificities of anti-AChR antibodies in sera of MG patients with and without thymoma.

Cell cycle dependent distribution of the proliferation-associated Ki-67 antigen in human embryonic lung cells.

Abstract: The cell cycle-dependent distribution of the proliferation-associated Ki-67 antigen has been evaluated immunocytochemically in L-132 human fetal lung cells. The cells were synchronized and cell cycle phases were determined: G1 = 6.7 h, S = 5.4 h, G2 = 8.5 h and mitosis = 1.3 h. The Ki-67 patterns were strictly correlated with the cell cycle phases. In late G1-phase, Ki-67 antigen was present only in the perinucleolar region. In the S-phase, Ki-67 staining was found homogeneously in the karyoplasm and in the perinucleolar region. G2-phase cells contained a finely granular Ki-67 staining in the karyoplasm with Ki-67-positive specks and perinucleolar staining. In early mitotic cells (pro- and metaphase) an intense perichromosomal Ki-67 staining was observed in addition to a homogeneously stained karyoplasm in prophase, and cytoplasm in metaphase. During ana- and telophase the Ki-67 antigen disappeared rapidly. In resting cells there was no Ki-67 staining.

Association of DNA topoisomerase I and RNA polymerase I: a possible role for topoisomerase I in ribosomal gene transcription

Abstract: RNA polymerase I preparations purified from a rat hepatoma contained DNA topoisomerase activity. The DNA topoisomerase associated with the polymerase had an Mr of 110000, required Mg2+ but not ATP, and was recognized by anti-topoisomerase I antibodies. When added to RNA polymerase I preparations containing topoisomerase activity, anti-topoisomerase I antibodies were able to inhibit the DNA relaxing activity of the preparation as well as RNA synthesis in vitro. RNA polymerase II prepared by analogous procedures did not contain topoisomerase activity and was not recognized by the antibodies. The topoisomerase I: polymerase I complex was reversibly dissociated by column chromatography on Sephacryl S200 in the presence of 0.25 M (NH4)2SO4. Topoisomeras I was immunolocalized in the transcriptionally active ribosomal gene complex containing RNA polymerase I in situ. These data indicate that topoisomerase I and RNA polymerase I are tightly complexed both in vivo and in vitro, and suggest a role for DNA topoisomerase I in the transcription of ribosomal genes.