GM1 Inhibits Amyloid β-Protein-Induced Cytokine Release

The ganglioside GM1 is known to play a pivotal role in neuronal survival and/or regeneration. Recently it has been shown that GM1 binds tightly with membrane-bound amyloid protein (A) and prevents its conversion from a helical to a -sheet structure. To examine the potential physiological consequences of this binding, we studied the effect of GM1 on A-stimulated release of proinflammatory cytokines, such as interleukin (IL)-1, IL-6 and TNF-, using the human monocytic cell line, THP-1, as a model system. Treatment of THP-1 cells with A 1–40 or A 25–35 resulted in an increased cytokine release from these cells. However, treatment of A-activated THP-1 cells with GM1 and several other complex gangliosides, but not hematosides and neutral glycosphingolipids such as asialo-GM1 (GA1), lactosylceramide, and globoside, significantly decreased the cytokine release. In contrast, this effect was not observed for lipopolysaccharide (LPS)-activated and thrombin-activated THP-1 cells, indicating that the ganglioside effect is specific for A-induced cytokine release. A direct interaction between GM1 and A was demonstrated using the surface plasmon resonance technique. We found that GM1 ganglioside exhibited higher affinity for A 1–40 than GA1, suggesting that the sialic acid moiety of GM1 is necessary for its interaction with A. We conclude that the inhibitory effect of GM1 on A-induced cytokine release may reflect pre-existing abnormalities in membrane transport at the stage of amyloid formation and that GM1 may induce conformational changes in A, resulting in diminished fibrillogenesis and prevention of the inflammatory response of neuronal cells in Alzheimer's disease.     

Involvement of β 1,4 galactosyltransferase 1 and Galβ 1→4GlcNAc groups in human hepatocarcinoma cell apoptosis

1,4 galactosyltransferase 1 ( 1,4GT1) synthesizes Gal 14GlcNAc groups in N-linked sugar chains of animal glycoproteins, which have been demonstrated to play an important role in many biological events, including sperm-egg interaction, cell migration and mammalian embryonic development. In this study, the mRNA level of 1,4GT1 was found to increase greatly during the 7721 hepatocarcinoma cells apoptosis induced by cycloheximide. Ricinus Communis Agglutinin-I staining indicated generous increase of Gal 14GlcNAc groups during apoptosis. Further study showed that the 7721 hepatocarcinoma cells transiently transfected with 1,4GT1 were more susceptible to the apoptosis induced by cycloheximide. The increased susceptibility was in accordance to the transfection concentration of 1,4GT1, which also led to the increased Gal 14GlcNAc groups on the transfected cell surface. All the observations suggested that 1,4GT1 and Gal 14GlcNAc groups might be associated with the apoptosis of human hepatocarcinoma cells.     

The energy transmission in ATP synthase: From the γ-c rotor to the α3β3 oligomer fixed by OSCP-b stator via the βDELSEED sequence

ATP synthase (F₀F₁) is driven by an electrochemical potential of H⁺ (H⁺). F₀F₁ is composed of an ion-conducting portion (F₀) and a catalytic portion (F₁). The subunit composition of F₁ is ₃₃. The active ₃₃ oligomer, characterized by X-ray crystallography, has been obtained only from thermnophilic F₁ (TF₁). We proposed in 1984 that ATP is released from the catalytic site (C site) by a conformational change induced by the DELSEED sequence via -F₀. In fact, cross-linking of DELSEED to stopped the ATP-driven rotation of in the center of ₃₃. The torque of the rotation is estimated to be 420 pN·å from the H⁺ and H⁺-current through F₀F₁. The angular velocity () of is the rate-limiting step, because H⁺ increased theV _max of H⁺ current through F₀, but not theK _m (ATP). The rotational unit of F₀ (=ab₂c₁₀) is /5, while that in ₃₃ is 2/3. This difference is overcome by an analog-digital conversion via elasticity around DELSEED with a threshold to release ATP. The distance at the C site is about 9.6 å (2,8-diN₃-ATP), and tight Mg-ATP binding in ₃₃ was shown by ESR. The rotational relaxation of TF₁ is too rapid (=100 nsec), but the rate of AT(D)P-induced conformational change of ₃₃ measured with a synchrotron is close to . The ATP bound between the P-loop and E188 is released by the shift of DELSEED from RGL. Considering the viscosity resistance and inertia of the free rotor (-c), there may be a stator containing OSCP (= of TF₁) and F₀-d to hold free rotation of ₃₃.     

Regioselective alkylation of benzylβ-d-lactoside and its derivatives by stannylation

The preparation of benzyl 2,3,6,2,6-penta-O-benzyl--d-lactoside, which is a key intermediate for chemical synthesis of oligosaccharide components of glycosphingolipids, was achieved by an improved method. The 3-O-p-methoxybenzyl and 3-O-methyl derivatives were prepared from benzyl 2,3,6,2,6-penta-O-benzyl--d-lactoside through stannylation. By using benzyl -d-lactoside as starting material, benzyl 3-O-methyl-, 3-O-benzyl- and 3-O-p-methoxybenzyl--d-lactoside were regioselectively synthesized using the same procedure.     

β-Adrenoceptors in the Tree Shrew Brain. II. Time-Dependent Effects of Chronic Psychosocial Stress on [125I]Iodocyanopindolol Binding Sites

1. Stress is known to affect the functioning of the central noradrenergic system in a region-specific manner. The aim of the present investigation was to understand better the consequences of recurrent stressful experiences on central -adrenoceptors.2. Alterations in the central nervous -adrenoceptor system resulting from different periods of psychosocial stress (PSS) were analyzed in male tree shrews (Tupaia belangeri) which were submitted to subordination stress for varying time periods.3. In the first experiment, the whole number of -adrenoceptors was analyzed in the forebrains of subordinate animals and controls by in vitro autoradiography using ¹²⁵I-iodocyanopindolol (¹²⁵ICYP), while nonspecific binding of the radioligand to serotonin receptors was blocked with 100 M 5HT.4. PSS affects -adrenoceptors in a time-dependent manner. A decrease in receptor affinity occurred after just 21 days of PSS in cortical areas and in the hippocampus, indicating stress effects on the conformation of -adrenoceptors. After 30 days of PSS,the numbers of -adrenoceptors were significantly decreased in several cortical regions and in the olfactory area.5. In the second experiment, we investigated the influence of PSS on both ₁- and ₂-adrenoceptors separately. ¹²⁵ICYP binding was quantified in the presence of either ICI188.551 to block ₂-adrenoceptors or in the presence of CGP20712A to block ₁-adrenoceptors.6. After 2, 10, 21,and 28 days of PSS, it become apparent that the two -adrenoceptor subtypes are regulated independently. ₁-Adrenoceptors were transiently down-regulated after 2 days of PSS in the prefrontal cortex and in the olfactory area and were decreased after 28 days of PSS in the parietal cortex and the hippocampus. A transient up-regulation of ₁-adrenoceptors occurred in the pulvinar nucleus after 10 days of PSS. ₂-Adrenoceptors were transiently down-regulated after 2 days of PSS in the prefrontal cortex and up-regulated in the pulvinar nucleus after 28 days of PSS.7. These data demonstrate that chronic psychosocial stress in subordinate tree shrews leads to time-dependent changes in the central nervous -adrenoceptors system.8. The high regional variability in stress-induced -adrenoceptor regulation is supposed to be due to the complex mechanisms of intracellular -adrenoceptor sequestration, which includes down-regulation and/or reinsertion of receptors into the plasma membrane. These mechanisms may be important components of the regulatory apparatus which enables the individual to adapt to situations of recurrent stressful experiences by balancing the central nervous adrenoceptor number.     

The Role of the Amino-Terminal β-Barrel Domain of the α and β Subunits in the Yeast F1-ATPase

The crystal structure of mitochondrial F₁-ATPase indicatesthat the and subunits fold into a structure defined by threedomains: the top -barrel domain, the middle nucleotide-binding domain,and the C-terminal -helix bundle domain (Abraham et al.1994); Bianchet et al., 1998). The -barrel domains of the and subunits form a crown structure at the top ofF₁, which was suggested to stabilize it (Abraham et al.1994). In this study. the role of the -barrel domain in the and subunits of the yeast Saccharomyces cerevisiae F₁,with regard to its folding and assembly, was investigated. The -barreldomains of yeast F₁ and subunits were expressedindividually and together in Escherichia coli. When expressedseperately, the -barrel domain of the subunit formed a largeaggregate structure, while the domain of the subunit waspredominately a monomer or dimer. However, coexpression of the -barreldomain of subunit domain. Furthermore, the two domains copurified incomplexes with the major portion of the complex found in a small molecularweight form. These results indicate that the -barrel domain of the and subunits interact specifically with each other and thatthese interactions prevent the aggregation of the -barrel domain of the subunit. These results mimic in vivo results and suggest thatthe interactions of the -barrel domains may be critical during thefolding and assembly of F₁.     

Determination of Hα,Hβ and Hβ,C′ coupling constants in13C-labeled proteins

Summary A sensitive method to assign H protons stereospecifically as well as to determine rotamer populations about ₁, in two 3D experiments is presented. The SOFT-HCCH-COSY experiment allowed us to measure the³J(H,C) couplings, using constant time evolution of C in t₂ and C_aliphatic-selective decoupling during t₃. The SOFT-HCCH-E.COSY experiment allowed us to measure the³J(H,H) couplings, using constant time evolution of C in t₂, a small flip angle¹H excitation pulse in the second mixing time, and double-band-selective decoupling (aliphatic and carbonyl carbons) during t₃. The method was applied to ribonuclease T₁.     

Synthesis of Galβ1-3GlcNAc and Galβ1-3GlcNAcβ-SEt by an enzymatic method comprising the sequential use of β-galactosidases from bovine testes andEscherichia coli

Gal1-3GlcNAc (1) and Gal1-3GlcNAc-SEt (2) were synthesized on a 100 mg scale by the transgalactosylation reaction of bovine testes -galactosidase with lactose as donor andN-acetylglucosamine and GlcNAc-SEt as acceptors. In both cases the product mixtures contained unwanted isomers and were treated with -galactosidase fromEscherichia coli which has a different specificity, under conditions favouring hydrolysis, yielding besides the desired products, monosaccharides and traces of trisaccharides. The products were purified to >95% by gel filtration, with a final yield of 12% of 1 and 17% of 2, based on added acceptor. In a separate experiment Gal1-6GlcNAc-SEt (3) was synthesized by the transglycosylation reaction using -galactosidase fromEscherichia coli. No other isomers were detected. Compound 3 was purified by HPLC.     

Primary structure of hemoglobin β-chain fromColumba livia (gray wild pigeon)

Primary structure of -chain of pigeon is presented. It was determined by amino acid sequence analysis of intact -chain and its peptides obtained by the enzymatic and chemical cleavage. Comparison of amino acid sequence of the chain with other available data shows 14 Ile, 61 Lys, and 113 Ile as residues specific to pigeon. One important replacement at ₁₁ contact is 55 MetSer.     

Effect of busulphan treatment and elevated temperature on the expression of the β-pol gene in rat testis

Changes in the expression pattern of the DNA polymerase gene during inhibition of spermatogenesis by busulphan and by temperature (artificial cryptorchidism) have been studied. Transient arrest of spermatogenesis in two-month-old rats after injection of a single dose of busulphan (10 mg/kg) resulted in parallel but transient decrease in the 1.4 kb of -pol mRNA level to an undetectable value, followed by its reappearance after resumption of spermatogenesis. An artificial cryptorchidism also caused a drastic decrease of -pol mRNA level. Both results as well as morphological examination of testis after busulphan injection and artificial cryptochidism revealed that spermatocytes and spermatids represent the testicular cell fraction containing the elevated amount of -pol mRNA. Involvement of DNA polymerase in meiotic recombination is discussed.     

Fromβ-lactams toα- andβ-amino acid derived peptides

Summary The potential of-lactams as intermediates for the access to- and-amino acid-derived peptides is shortly reviewed, with major focus on the technologies developed in our group. The two general strategies lie, on one side, in the oxidative ring expansion of 3-hydroxy-lactams toN-carboxy-amino acid anhydrides or Leuch's anhydrides and subsequent coupling with-amino acid esters and, on the other side, in the nucleophilic ring opening ofN-Boc--lactams. Both approaches have been successfully applied to the synthesis of,-diamino acid,-amino--hydroxy acid, polyhydroxylated-amino acid,,-disubstituted-amino acid,-amino acid,-amino--hydroxy acid and,-disubstituted-amino acid derived peptides. Because of the mild reaction conditions needed for the above transformations and the highly stereoselective procedures employed for the construction of the starting-lactam ring, the whole process allows the production of optically pure final products.     

A small variance in the antigenicity but not function of recombinant β-lactoglobulin purified from the culture supernatant of transformed yeast cells

We purified recombinant bovine -lactoglobulin (r-LG) from the culture supernatant of transformed yeast and investigated whether r-LG maintained the functional ability and antigenicity of native -LG. Immunostaining following gel electrophoresis and reversed-phase high-performance liquid chromatography confirmed that r-LG was purified homogeneously. r-LG showed almost the same retinol-binding ability as native -LG purified from bovine milk. However, affinities of two anti--LG monoclonal antibodies (mAbs) to r-LG were different from those to native -LG, although three other mAbs bound these two proteins equally. Since our panel of five mAbs has been previously shown to be able to detect structural changes occurring in -LG, this variance in antigenicity can be attributed to conformational differences between r-LG and native -LG. Then, we studied which step in the production and purification procedure was responsible for altering the antigenicity of r-LG. Bovine milk native -LG was added to several steps in this procedure and purified in the same manner as r-LG. The results suggested that incubation in the yeast culture had adverse effects on maintaining the antigenicity of this recombinant protein. We conclude from these results that even if no difference between the native and recombinant proteins can be detected by functional analysis, some subtle conformational change which can be distinguished by mAbs may be incorporated into the recombinant protein during its production and ultimately cause a different immune reaction in vivo.Abbreviations -LG, -lactoglobulin; r-LG, recombinant -LG; PBS, phosphate-buffered saline; PBS-Tween, PBS containing 0.05% Tween 20; ELISA, enzyme-linked immunosorbent assay.     

Cloning and expression of a β-glycosidase gene from Thermus thermophilus. Sequence and biochemical characterization of the encoded enzyme

A 3.2 kilobase pair DNA fragment from Thermus thermophilus HB27 coding for a -galactosidase activity was cloned and sequenced. A gene and a truncated open reading frame orf1 encoding respectively a -glycosidase (tt-gly) and probably a sugar permease were located directly adjacent to each other. The deduced aminoacid sequence of the enzyme Tt-gly showed strong identity with those of -glycosidases belonging to the glycosyl hydrolase family 1. The enzyme was overexpressed in Escherichia coli and was purified by a two-step purification procedure. The recombinant enzyme is monomeric with a molecular mass of 49-kDa. It catalyzes the hydrolysis of -D-galactoside, -D-glucoside and -D-fucoside derivatives. However, the kcat/Km ratio is much higher for p-nitrophenyl--D-glucoside and p-nitrophenyl--D-fucoside than for p-nitrophenyl--D-galactoside. The specificity towards linkage positions of the disaccharides tested decreased in the following order: 1-3 (100%) < 1-2 (71%) < 1-4 (40%) < 1-6 (10%). Tt-gly is a thermostable enzyme displaying an optimum temperature of 88°C and a half life of 10 min at 90°C. It performs transglycosylation reactions at high temperature with a yield exceeding 63% for transfucosylation reactions. On the basis of this work, the enzyme appears to be an attractive tool in the synthesis of fucosyl adducts and fucosyl sugars.     

β-Adrenoceptors in the Tree Shrew Brain. I. Distribution and Characterization of [125I]Iodocyanopindolol Binding Sites

1. The number and distribution pattern of -adrenergic receptors in the brain have been reported to be species specific. The aim of the present study was to describe binding of the -adrenoceptor ligand [¹²⁵I]iodocyanopindolol in the brain of the tree shrew (Tupaia belangeri), a species which provides an appropriate model for studies of psychosocial stress and its consequences on central nervous processes.2. ¹²⁵I-Iodocyanopindolol (¹²⁵ICYP) labeling revealed a high degree of nonspecific binding, which was due mainly to interactions of this ligand with serotonin binding sites. For a quantitative evaluation of ₁- and ₂-adrenoceptors, serotonin binding sites had to be blocked by 100 M 5HT.3. Binding of the radioligand to ₁- and ₂-adrenoceptors was characterized using the ₁-specific antagonist CGP20712A and the ₂-specific antagonist ICI118.551. ₁-adrenoceptor binding is present in the whole brain, revealing low receptor numbers in most brain regions (up to 1.5 to 2.7 fmol/mg). A slight enrichment was observed in cortical areas (lateral orbital cortex: 4.0±0.7 fmol/mg) and in the cerebellar molecular layer (8.7±1.0 fmol/mg).4. Competition experiments demonstrated high- and low-affinity binding sites with considerable variations in K _i values for CGP20712A, showing that various affinity states of ₁-adrenoceptors are present in the brain (K _i: 0.61 nM to 67.1 M). In the hippocampus, only low-affinity ₁-adrenoceptors were detected (K _i: 1.3±0.2 M). Since it is known that ¹²⁵ICYP labels not only membrane bound but also internalized -adrenoceptors, it can be assumed that the large population of the low-affinity sites represents internalized receptors which may be abundant due to a high sequestration rate.5. High numbers of ₂-adrenoceptors are present in only a few brain structures of tree shrews (external layer of the olfactory bulb, 15.8±2.0 fmol/mg; claustrum, 19.3±1.5 fmol/mg; anteroventral thalamic nucleus, 19.4±1.5 fmol/mg; cerebellar molecular layer, 55.0±4.3 fmol/mg). Also for this class of -adrenoceptors, high- and low-affinity binding sites for the ₂-selective antagonist ICI118.551 were observed, indicating that ¹²⁵ICYP labels membrane bound and internalized ₂-adrenoceptors. Only in the cerebellar molecular layer was a high percentage of high-affinity ₂-adrenoceptors detected (K _i for ICI118.551 was 1.8±0.3 nM for 90% of the receptors).6. In conclusion, ₁- and ₂-adrenoceptor binding can be localized and quantified by in vitro receptor autoradiography in the brains of tree shrews when serotonergic binding sites are blocked. Modulatory effects of long-term psychosocial conflict on the central nervous -adrenoceptor system in male tree shrews are described in the following paper.     

Association of β<Subscript>2</Subscript>-microglobulin with the α<Subscript>3</Subscript> domain of H-2D<Superscript>b</Superscript> heavy chain

MHC class I molecules are heterotrimeric complexes composed of heavy chain, ₂-microglobulin (₂m) and short peptide. This trimeric complex is generated in the endoplasmic reticulum (ER), where a peptide loading complex (PLC) facilitates transport from the cytosol and binding of the peptide to the preassembled ER resident heavy chain/₂m dimers. Association of mouse MHC class I heavy chain with ₂m is characterized by allelic differences in the number and/or positions of amino acid interactions. It is unclear, however, whether all alleles follow common binding patterns with minimal contributions by allele-specific contacts, or whether essential contacts with ₂m are different for each allele. While searching for the PLC binding site in the ₃ domain of the mouse MHC class I molecule H-2D^b, we unexpectedly discovered a site critical for binding mouse, but not human, ₂m. Interestingly, amino acids in the corresponding region of another MHC class I heavy chain allele do not make contacts with the mouse ₂m. Thus, there are allelic differences in the modes of binding of ₂m to the heavy chain of MHC class I.     

Expression sites and developmental regulation of genes encoding (1→3,1→4)-β-glucanases in germinated barley

Expression sites of genes encoding (13,14)--glucan 4-glucanohydrolase (EC 3.2.1.73) have been mapped in germinated barley grains (Hordeum vulgare L.) by hybridization histochemistry. A³²P-labelled cDNA (copy DNA) probe was hybridized to cryosections of intact barley grains to localize complementary mRNAs. No mRNA encoding (13,14)--glucanase is detected in ungerminated grain. Expression of (13,14)--glucanase genes is first detected in the scutellum after 1 d and is confined to the epithelial layer. At this stage, no expression is apparent in the aleurone. After 2 d, levels of (13,14)--glucanase mRNA decrease in the scutellar epithelium but increase in the aleurone. In the aleurone layer, induction of (13,14)--glucanase gene expression, as measured by mRNA accumulation, progresses from the proximal to distal end of the grain as a front moving away from, and parallel to, the face of the scutellum.Abbreviations cDNA copy DNA - RNase ribonuclease     

Suppression of fungal β-glucan-induced plant defence in soybean (Glycine max L.) by cyclic 1,3-1,6-β-glucans from the symbiont Bradyrhizobium japonicum

The production of antimicrobial phytoalexins is one of the best-known inducible defence responses following microbial infection of plants or treatment with elicitors. In the legume soybean (Glycine max L.), 1,3-1,6--glucans derived from the fungal pathogen Phytophthora sojae have been identified as potent elicitors of the synthesis of the phytoalexin, glyceollin. Recently it has been reported that during symbiotic interaction between soybean and the nitrogen-fixing bacterium Bradyrhizobium japonicum USDA 110 the bacteria synthesize cyclic 1,3-1,6--glucans. Here we demonstrate that both the fungal and the bacterial -glucans are ligands of -glucan-binding sites which are putative receptors for the elicitor signal compounds in soybean roots. Whereas the fungal -glucans stimulate phytoalexin synthesis at low concentrations, the bacterial cyclic 1,3-1,6--glucans appear to be inactive even at relatively high concentrations. Competition studies indicate that increasing concentrations of the bacterial 1,3-1,6--glucans progressively inhibit stimulation of phytoalexin synthesis in a bioassay induced by the fungal 1,3-1,6--glucans. Another type of cyclic -glucan, a 1,2--glucan from Rhizobium meliloti, that does not nodulate on soybean, seems to be inactive as elicitor and as ligand of the -glucan-binding sites. These results may indicate a novel mechanism for a successful plant-symbiont interaction by suppressing the plant's defence response.Abbreviations HG-APEA 1-[2-(4-aminophenyl)ethyl]amino-l-[hexaglucosyl]deoxyglucitol - HG-AzPEA l-[2-(4-azidophenyl)-ethyl]amino-l-[hexaglucosyl]deoxyglucitol - IC₅₀ concentration for half-maximal displacement We thank Ines Arlt for excellent technical assistance. This work was supported by the Deutsche Forschungsgemeinschaft (SFB 369), the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, Fonds der Chemischen Industrie (J.E.), and USDA CSRS NRI Competitive Research grant 93373059233 (A.A.B.).     

One-bond carbon-proton coupling constants: Angular dependence in β-linked oligosaccharides

Summary The angular dependence of¹J_C,H in model compounds related to -linked oligosaccharides has been established by FPT INDO quantum chemical calculations. Values calculated for models of (1 1)-, (1 2)-, (1 3)- and (1 4)-linked disaccharides were compared, and the effect of the orientation of HO-2 elucidated. The angular dependence of¹J_C,H on the torsional angles ^H and ^H and the solvent dielectric constant (s) was characterized in the form:¹J_C,H = A cos2+B cos + C sin2 + D since + E + Fe. The¹J_C,H values, measured by DEPT methods for C-1-H-1 and C-X-H-X in cellobiose, cyclic trisaccharide and hexopyranoses were used to adjust the calculated angular dependences. Based on the occurrence of the conformers for agarobiose, neoagarobiose, mannobiose and methyl -xylobioside, the thermodynamically averaged <¹J_C,H > values were calculated. The results obtained (<¹J_C-1,H-1 > 162.4, <¹J_{C-4, H-4} > 147.6 Hz for methyl -xylobioside; <¹J_C-1,H-1 > 162.4 and <¹J_C-4,H-4] > 147.6 Hz for mannobiose; <¹J_C-1,H-1 > 162.8 Hz for neo agarobiose and <¹J_C-1,H-1 > 163.2 Hz for agarobiose) agree well with the experimental values of 162.7, 147.5, 160.4, 147.2, 160.9 and 165.7 Hz, respectively.     

Antimutagenic Role of Autonomous 3′ → 5′-Exonucleases

Original and published data on the antimutagenic role of autonomous 3 5-exonucleases (AE) are analyzed. AE are not bound covalently to DNA polymerases but are often involved in replicative complexes. AE overproduction in bacterial cells is accompanied by a sharp suppression of mutagenesis, whereas AE inactivation in bacteria and higher fungi results in the increase in mutation rates by two to three orders of magnitude. The combined action of AE and DNA polymerases substantially improves the fidelity of their functioning in vitro. The fidelity of nuclease-free DNA polymerases and increases by two to three orders of magnitude in the presence of AE. The fidelity of moderately processive DNA polymerase I increases by two orders of magnitude, and that of highly processive DNA polymerase increases by a factor of 5–10, although both these polymerases possess their own 3 5-exonucleolytic activity. In biochemical experiments, AE was shown to participate directly in the correction of errors made by DNA polymerase I. The presence of AE in multienzyme DNA polymerase complexes increases their fidelity by a factor of 5–10. A model of extrinsic proofreading by AE in DNA biosynthesis is proposed. An investigation of thirty objects from all three kingdoms of life (from archaea and bacteria to mammals, including humans) has shown that AE account for 30–90% of the total cellular 3 5-exonucleolytic activity. Therefore, AE increase significantly the intracellular ratio of 3 5-exonuclease to DNA polymerase activities in a wide phylogenetic variety of species, which always leads to the increasing fidelity of DNA biosynthesis.