Analysis of the possible helical structures of nucleic acids and polynucleotides. Application of (n-h) plots.

The two helical parameters n and h where n is the number of nucleotide residues per turn and h is the height per nucleotide residue have been evaluated for single stranded helical polynucleotide chains comprising C(3') -endo and C(2') endo class of nucleotides. The helical parameters are found to be especially sensitive to the C(4')-C(3') (sugar pucker) and the C(4')-C(5') torsions. The (n-h) plots display only one important helix forming domain for each class of nucleotides characterized by the sugar pucker and the C(4')-C(5') torsion. A correlation between the (n-h) plots and the known RNA (A,A') and DNA (A,B,C) helical forms has been established. It is found that all forms of helices except the C-DNA possess a favorable combination of P-O torsions. The analysis of the (n-h) plots suggests that C-DNA can have a conformation very similar to B-DNA. Although the (n-h) plots predict the stereochemical possibility of both right-handed and left-handed helices, nucleic acids apparently prefer right-handed conformation because of the energetics associated with the sugar-phosphate backbone and the base.     

Molecular structure of deoxyadenylyl-3'-methylphosphonate-5'-thymidine dihydrate, (d-ApT x 2H2O), a dinucleoside monophosphate with neutral phosphodiester backbone. An X-ray crystal study.

dApT, a modified deoxyribose dinucleoside phosphate with an uncharged methylphosphonate group, crystallizes as dihydrate in space group P2(1)2(1)2, a = 9.629(3), b = 20.884(6) and c = 14.173(4)A, Z = 4. The structure has been determined using 2176 X-ray diffractometer reflections and refined to a final R of 0.105. Torsion angles about P-O(5') and P-O(3') bonds are -91.8 degrees and 117.8 degrees. The former is in the normal (-)gauche range while the latter is eclipsed. Bases are oriented anti, the sugar of adenosine is puckered 2T3 (C(2')endo) whereas that of thymidine displays puckering disorder with major and minor occupancy sites. Major site is a half-chair 2T (C(2')endo-C(1')exo) and minor site an envelope 3T2 (C(3(1)endo). Adenine and thymine bases of symmetry related molecules form reversed Hoogsteen type base pairs, water molecules are disordered in the crystal lattice.     

Crystal structures of B-DNA with incorporated 2'-deoxy-2'-fluoro-arabino-furanosyl thymines: implications of conformational preorganization for duplex stability.

The fundamental conformational states of right-handed double helical DNA, the A- and B-forms, are associated with distinct puckers of the sugar moieties. The furanose conformation itself is affected by the steric and electronic nature of the ring substituents. For example, a strongly electronegative substituent at the C2' position, such as in the 2'-deoxy-2'-fluoro ribo furanosyl analogue, will drive the conformational equilibrium towards the C3'- endo type (north). Conversely, the 2'-deoxy-2'-fluoro arabino furanosyl modification with opposite stereochemistry at C2' appears to have a preference for a C2'- endo type pucker (south). Incorporation of 2'-fluoroarabinofuranosyl thymines was previously shown to enhance the thermodynamic stability of B-DNA duplexes. We have determined the crystal structures of the B-DNA dodecamer duplexes [d(CGCGAASSCGCG)]2and [d(CGCGAASTCGCG)]2with incorporated 2'-deoxy-2'-fluoroarabinofuranosyl thymines S (south) at 1.55 A resolution. In the crystal structures, all S residues adopt an O4'- endo conformation (east), well compatible with an overall B-form duplex geometry. In addition to the increased rigidity of S nucleosides, a clathrate-like ordered water structure around the 2'-fluorines may account for the observed larger thermodynamic stability of DNA duplexes containing 2'-deoxy-2'-fluoroarabino thymidines.     

Acoustical investigation of poly(dA).poly(dT), poly[d(A-T)], poly(A).poly(U) and DNA hydration in dilute aqueous solutions.

Apparent molar adiabatic compressibilities and apparent molar volumes of poly[d(A-T)].poly[d(A-T)], poly(dA).poly(dT), DNA and poly(A).poly(U) in aqueous solutions were determined at 1 degree C. The change of concentration increment of the ultrasonic velocity upon replacing counter ion Cs+ by the Mg2+ ion was also determined for these polymers. The following conclusions have been made: (1) the hydration of the double helix of poly(dA).poly(dT) is remarkably larger than that of other polynucleotides; (2) the hydration of the AT pair in the B-form DNA is larger than that of the GC pair; (3) the substitution of Cs+ for Mg2+ ions as counter ions results in a decrease of hydration of the system polynucleotide plus Mg2+, and (4) the magnitude of this dehydration depends on the nucleotide sequence; the following rule is true: the lesser is a polynucleotide hydration, the larger dehydration upon changing Cs+ for Mg2+ ions in the ionic atmosphere of polynucleotide.     

Conformational flexibility in DNA structure and its implications in understanding the organization of DNA in chromatin.

X-ray crystallographic studies of drug-nucleic acid crystalline complexes have suggested that DNA first bends or 'kinks' before accepting an intercalative drug or dye. This flexibility in DNA structure is made possible by altering the normal C2' endo deoxyribose sugar puckering in B DNA to a mixed sugar puckering pattern of the type C3' endo (3'-5') C2' endo and partially unstacking base pairs. A kinking scheme such as this would require minimal sterochemical rearrangement and would also involve small energies. This has prompted us to ask more generally if a conformational change such as this could be used by proteins in their interactions with DNA. Here we describe an interesting superhelical DNA structure formed by kinking DNA every ten base pairs. This structure may be used in the organization of DNA within the nucleosome structure in chromatin.     

Reassessment of structural characteristics of the d(CGCG)2:actinomycin D complex from complete 1H and 31P NMR

Complexes formed between Actinomycin D (ActD) and the tetranucleotides d(AGCT)2 and d(CGCG)2 were studied in detail by one and two-dimensional 1H and 31P NMR. The 31P two dimensional chemical exchange experiment, at room temperature on saturated complexes (1:1), showed unambiguously that the asymmetrical phenoxazone ring binds to the unique GC site under the two possible orientations in the d(AGCT)2 tetranucleotide but adopts a single orientation in the d(CGCG)2 tetranucleotide. For the d(CGCG)2:Act D saturated complex, complete assignments of all protons and phosphorus signals of the two-nucleotide strands, as well as of the two cyclic pentapeptide chains has allowed us to study in details the conformational features of the complex from NOE and coupling constants analysis. The tetranucleotide remains in a right-handed duplex, but the sugar puckers are modified for residues at the intercalation site. A uniform C2' endo pucker is observed for residues on the strand facing the quinoid side of the phenoxazone ring while a C2' endo-C3-endo equilibrium about 60% of C2' endo is proposed for the two residues on the strand facing the benzenoid side of the phenoxazone ring. In contrast to previous studies on ActD-DNA interactions, we have been able to measure the 3J phosphorus-proton coupling constants at the intercalation site but also adjacent to it, showing that 31P chemical shifts are not simply related to the backbone conformation. Molecular mechanics calculations, using empirical distances deduced from NOE effects as restrained distances during minimizations, led to a model differing mainly from those previously published by orientation of the N methyl groups of both N-Methyl-Valines.     

Structure of poly 8-bromoadenylic acid; conformational studies by CPF energy calculations.

Poly 8-bromoadenylic acid [poly(BBrA)] is the only known all-syn polynucleotide. It shows a helix-coil transition with a melting curve centred around 55 degrees C. Energy calculations based on classical potential functions have been used to explore the three-dimensional structure of this polymer in helix and random coil. It is concluded that the ordered state is a helix of two parallel strands with a two-fold rotation axis, and the duplex is stabilised by hydrogen bonds involving N1 and H6. Each strand has a conformation with C3' endo geometry, phi' = 216 degrees, omega' = 280 degrees, omega = 294 degrees, phi = 179 degrees, chi = 243 degrees and psi = 57 degrees. Such a conformation leads to approximately 8 nucleotide units per turn of the helix and an axial rise of 3.9A degrees. The structure of poly(8BrA) has been compared with that of the related polymer poly(A) which forms a double helical structure in acidic conditions with bases in the anti conformation and with interstrand hydrogen-bonds between N7 and H6. This is the first time that a specific geometrical model of a novel polynucleotide structure has been produced initially by potential energy calculations, though such calculations on a number of known structures have been reported previously.     

Structural characteristics of 2'-O-(2-methoxyethyl)-modified nucleic acids from molecular dynamics simulations.

The structure and physical properties of 2'-sugar substituted O -(2-methoxyethyl) (MOE) nucleic acids have been studied using molecular dynamics simulations. Nanosecond simulations on the duplex MOE[CCAACGTTGG]-r[CCAACGUUGG] in aqueous solution have been carried out using the particle mesh Ewald method. Parameters for the simulation have been developed from ab initio calculations on dimethoxyethyl fragments in a manner consistent with the AMBER 4.1 force field database. The simulated duplex is compared with the crystal structure of the self-complementary duplex d[GCGTATMOEACGC]2, which contains a single modification in each strand. Structural details from each sequence have been analyzed to rationalize the stability imparted by substitution with 2'- O -(2-methoxyethyl) side chains. Both duplexes have an A-form structure, as indicated by several parameters, most notably a C3' endo sugar pucker in all residues. The simulated structure maintains a stable A-form geometry throughout the duration of the simulation with an average RMS deviation of 2.0 A from the starting A-form structure. The presence of the 2' substitution appears to lock the sugars in the C3' endo conformation, causing the duplex to adopt a stable A-form geometry. The side chains themselves have a fairly rigid geometry with trans , trans , gauche +/- and trans rotations about the C2'-O2', O2'-CA', CA'-CB' and CB'-OC' bonds respectively.     

A new D-DNA form of poly(dA-dT).poly(dA-dT): an A-DNA type structure with reversed Hoogsteen pairing

The D-DNA double helix model of poly(dA-dT).poly(dA-dT) proposed in the literature is not in accordance with some notable experimental facts and physicochemical conditions to which it is related. Thus, the fibre X-ray diffraction pattern of D-DNA obtained at a relative humidity lower than that giving the A-DNA form is singularly not taken into account when one assumes that there is only one D structure of B-DNA type. We rather suggest that there are actually two different forms of D-DNA, namely D(A) which partakes in the D-A-B transitions and D(B) associated with the D-B change of conformation. Although these two DNA structures have the same helical parameters (pitch and number of residues per turn), in agreement with X-ray data, their detailed conformations are considerably different. Whereas D(B) is indeed the structure generally defined as D-DNA, a critical analysis based on a comparison between different possible DNA double helices leads us to propose dihedral angles, a set of atomic coordinates and a stereo view of another new form of D-DNA, the D(A) structural model. It is a right-handed double helix with a dinucleotide as the repeat unit. The furanose rings are of the A-DNA type (C3' endo) and the bases are hydrogen bonded according to the reversed Hoogsteen pairing. Such a disposition renders the D(A) model unsuitable for poly(dI-dC).poly(dI-dC), the other alternating polynucleotide observed in the D(B) structure. The consistency of these two different D-DNA structures of poly(dA-dT).poly(dA-dT) with the general aspects of hydration and helix-helix transitions of DNA, as well as with the conformational variability of AT base sequences, is discussed.     

High-resolution NMR study of a synthetic DNA-RNA hybrid dodecamer containing the consensus pribnow promoter sequence: d(CGTTATAATGCG).r(CGCAUUAUAACG)

The nonsymmetrical double-helical hybrid dodecamer d(CGTTATAATGCG).r(CGCAUUAUAACG) was synthesized with solid-phase phosphoramidite methods and studied by high-resolution 2D NMR. The imino protons were assigned by one-dimensional nuclear Overhauser methods. All the base protons and H1', H2', H2", H3', and H4' sugar protons of the DNA strand and the base protons, H1', H2', and most of the H3'-H4' protons of the RNA strand were assigned by 2D NMR techniques. The well-resolved spectra allowed a qualitative analysis of relative proton-proton distances in both strands of the dodecamer. The chemical shifts of the hybrid duplex were compared to those of the pure DNA double helix with the same sequence (Wemmer et al., 1984). The intrastrand and cross-strand NOEs from adenine H2 to H1' resonances of neighboring base pairs exhibited characteristic patterns that were very useful for checking the spectral assignments, and their highly nonsymmetric nature reveals that the conformations of the two strands are quite different. Detailed analysis of the NOESY and COSY spectra, as well as the chemical shift data, indicate that the RNA strand assumes a normal A-type conformation (C3'-endo) whereas the DNA strand is in the general S domain but not exactly in the normal C2'-endo conformation. The overall structure of this RNA-DNA duplex is different from that reported for hybrid duplexes in solution by other groups (Reid et al., 1983a; Gupta et al., 1985) and is closer to the C3'-endo-C2'-endo hybrid found in poly(dA).poly(dT) and poly(rU).poly(dA) in the fiber state (Arnott et al., 1983, 1986).     

The concentration dependence of the right to left conformational transition in natural DNA identified by Raman spectroscopy

The classical and resonance Raman spectra of DNA from Chicken Erythrocytes have been obtained for different DNA concentrations in solution with low and high ionic strengths. The classical Raman spectra of 30 mg/ml DNA solutions were measured in varying the sodium chloride concentration from 0.1 to 4.5 M NaCl. An increase in the salt content of the solution leads to spectral changes in the 600-700 cm-1 region, indicating a C2' endo/anti to C3' endo/syn conformational transition of the purine residues. Other changes around 840 cm-1, due to the antisymmetrical stretching vibration of the PO2 group, are also detected: they were characteristic for the B----Z transition in model systems such as poly(dG-dC).poly(dG-dC). The resonance Raman spectra of low (1 mg/ml) and high (30 mg/ml) concentrated DNA solutions were obtained with low (0.1 M) and high (4.5 M) NaCl contents, in using a 284 nm excitation wavelength. No change was observed in the intensities and band positions in the low and high salt solutions of low concentrated DNA. Thus it is assumed that the DNA structure remains unchanged whatever the salt concentration for low concentrated DNA. In contrast, great modifications of the intensities and positions of some lines were found in the spectra of high DNA concentration solution when the NaCl content is increased up to 4.5 M: these changes resemble to some extent those observed in the study of B----Z transition of several polynucleotide model compounds. It is assumed that the right-handed to left-handed conformational transition may occur in certain sections of natural DNA, likely containing alternating purine-pyrimidine sequences, when the DNA concentration is sufficiently important.     

Binding of Hoechst 33258 and its Derivatives to DNA

Abstract

In the present work, we employed UV-VIS spectroscopy, fluorescence methods, and circular dichroism spectroscopy (CD) to study the interaction of dye Hoechst 33258, Hoechst 33342, and their derivatives to poly[d(AT)]·poly[d(AT)], poly(dA)·poly(dT), and DNA dodecamer with the sequence 5′-CGTATATATACG-3′. We identified three types of complexes formed by Hoechst 33258, Hoechst 33342, and methylproamine with DNA, corresponding to the binding of each drug in monomer, dimer, and tetramer forms. In a dimer complex, two dye molecules are sandwiched in the same place of the minor DNA groove. Our data show that Hoechst 33258, Hoechst 33342, and methylproamine also form complexes of the third type that reflects binding of dye associates (probably tetramers) to DNA. Substitution of a hydrogen atom in the ortho position of the phenyl ring by a methyl group has a little effect on binding of monomers to DNA. However it reduces strength of binding of tetramers to DNA. In contrast, a Hoechst derivative containing the ortho-isopropyl group in the phenyl ring exhibits a low affinity to poly(dA)·poly(dT) and poly[d(AT)]·poly[d(AT)] and binds to DNA only in the monomer form. This can be attributed to a sterical hindrance caused by the ortho-isopropyl group for side-by-side accommodation of two dye molecules in the minor groove. Our experiments show that mode of binding of Hoechst 33258 derivatives and their affinity for DNA depend on substituents in the ortho position of the phenyl ring of the dye molecule. A statistical mechanical treatment of binding of Hoechst 33258 and its derivatives to a polynucleotide lattice is described and used for determination of binding parameters of Hoechst 33258 and its derivatives to poly[d(AT)]·poly[d(AT)] and poly(dA)·poly(dT).     

Conformation in aqueous medium of the neutral, protonated and anionic forms of 9-beta-D-arabinofuranosyladenine.

Proton magnetic resonance spectroscopy was employed to study the solution conformations of the neutral, protonated and dissociated forms of the therapeutically active 9-beta-D-arabinofuranosyladenine (araA). In particular, in strongly basic medium, increasing alkalinity led to pronounced changes in chemical shifts and coupling constants of some pentose protons, due to ionization of the pentose hydroxyls, especially the 2'-OH. The neutral form of araA may be characterized as approx. 25% C(2')endo and approx. 60% gauche-gauche, hence somewhat different from that of the therapeutically active 1-beta-D-arabinofuranosylcytosine (araC). By contrast, the conformations of the anionic forms of both of these are identical, predominantly (greater than 80%) C(2')endo and gauche-gauche. With the aid of the 3'-O-methyl derivatives of araA and araC, where only the 2'-OH ionizes, and the accompanying conformational changes are similar, it follows that the conformation C(2')endo and gauche-gauche for all the foregoing is constrained to this form via a strong intramolecular hydrogen bond, viz. O(5')H...O(2')(-). The influence of the foregoing hydrogen bond on the chemical shifts of the adenine H(8) in the araA anion points to the existence of the latter in the form anti. A similar effect of the doubly ionized phosphate group on H(8) in 5'-araAMP shows the nucleotide to also prefer the form anti, as previously demonstrated for 5'-AMP. The conformations of the sugar rings of the neutral forms of araA and adenosine in aqueous medium differ appreciably, whereas in the solid state they are very similar. PMR spectroscopy is shown to be an effective method for following sugar hydroxyl dissociation. The extent of ionization of a given hydroxyl is provided by the resulting chemical shifts of neighbouring (geminal and vicinal) protons. When ionization is accompanied by a change in conformation, the process may be followed also by changes in proton-proton vicinal coupling constants.     

Effect of the strandedness of cofactor DNA on the activities of DNA-dependent ATPases B and C3 from FM3A cells

The requirements of cofactor DNA for DNA-dependent ATPases B and C3 were analyzed in detail. ATPase B and C3 required the presence of a polynucleotide for their activities. Among the DNAs tested, ATPase B showed a preference for poly(dT) as its cofactor. The other deoxyhomopolymers, except poly(dG) and heat-denatured DNA also were effective. The alternating polydeoxyribonucleotide, poly[d(A-T)] had an efficiency 23% that of heat-denatured DNA. Unlike ATPase B, ATPase C3 showed almost no activity with deoxyhomopolymers. The most effective cofactor for ATPase C3 so far tested is poly[d(A-T)]. Relatively high activity was obtained with heat-denatured DNA. The high activity of ATPase B with poly(dT) was reduced by the addition of poly(dA). The addition of noncomplementary homopolymers did not affect enzyme activity. ATPase C3 activity in the presence of 10 microM poly(dT) increased gradually with concentrations of poly(dA) up to 20 microM, after which it decreased. Almost no increase in activity was observed when noncomplementary homopolymers were added. The relatively high activity of ATPase C3 with heat-denatured DNA was suggested by its high sensitivity to ethidium bromide to be due to the double-stranded region in the heat-denatured DNA formed by self-annealing.     

Enzymatic analysis of isomeric trithymidylates containing ultraviolet light-induced cyclobutane pyrimidine dimers. II. Phosphorylation by phage T4 polynucleotide kinase

Phage T4 polynucleotide kinase (EC 2.7.1.78) proved incapable of catalyzing the phosphorylation of thymidylyl-(3'----5')-thymidine containing either a cis-syn-cyclobutane pyrimidine dimer (d-T less than p greater than T) or a 6-4'-[pyrimidin-2'-one]pyrimidine photoproduct (d-T[p]-T), and similarly the UV-modified compounds of (dT)3 bearing either photoproduct at their 5'-end (d-T less than p greater than TpT and d-T[p]TpT). In contrast, the 3'-structural isomers of these trinucleotides (d-TpT less than p greater than T and d-TpT[p]T) were phosphorylated at the same rate as the parent compound. These phosphorylatable lesion-containing oligonucleotides are quantitatively released from UV-irradiated poly(dA):poly(dT) by enzymatic hydrolysis with snake venom phosphodiesterase and alkaline phosphatase (Liuzzi, M., Weinfeld, M., and Paterson, M. C. (1989) J. Biol. Chem. 264, 6355-6363). By combining this digestion regimen with phosphorylation by polynucleotide kinase and [gamma-32P]ATP, pyrimidine dimers were quantitated at the fmol level following exposure of poly(dA):poly(dT) and herring sperm DNA to biologically relevant UV fluences. The rate of dimer induction in the synthetic polymer, approximately 10 dimers/10(6) nucleotides/Jm-2, was in close agreement with that obtained by conventional methods. Dimers were induced at one-fourth of this rate in the natural DNA. Further treatment of the phosphorylated oligonucleotides derived from irradiated herring sperm DNA with nuclease P1 released the labeled 5'-nucleotide, thus permitting analysis of the nearest-neighbor bases 5' to the lesions. We observed a ratio for pyrimidine-to-purine bases of almost 6:1, implicating tripyrimidine stretches as hotspots for UV-induced DNA damage.     

Circular dichroism studies of the interaction of a limited hydrolysate of T4 gene 32 protein with T4 DNA and poly[d(A-T)].poly[d(A-T)].

gp32 I is a protein with a molecular weight of 27 000. It is obtained by limited hydrolysis of T4 gene 32 coded protein, which is one of the DNA melting proteins. gp32 I itself appears to be also a melting protein. It denatures poly[d(A-T)].poly[d(A-T)] and T4 DNA at temperatures far (50-60 degrees C) below their regular melting temperatures. Under similar conditions gp32 I will denature poly[d(A-T).poly[d(A-T)] at temperatures approximately 12 degrees C lower than those measured for the intact gp32 denaturation. For T4 DNA gp32 shows no melting behavior while gp32 I shows considerable denaturation (i.e., hyperchromicity) even at 1 degree C. In this paper the denaturation of poly[d(A-T)].poly[d(A-T)] and T4 DNA by gp32 I is studied by means of circular dichroism. It appears that gp32 I forms a complex with poly[d(A-T)]. The conformation of the polynucleotide in the complex is equal to that of one strand of the double-stranded polymer in 6 M LiCl. In the gp32 I DNA complex formed upon denaturation of T4 DNA, the single-stranded DNA molecule has the same conformation as one strand of the double-strand T4 DNA molecule in the C-DNA conformation.     

Synthesis and characterization of poly[d(G-z5C)]. B-Z transition and inhibition of DNA methylase

Deoxy-5-azacytidine 5'-triphosphate was synthesized and used as a substrate for the enzymatic synthesis of the polynucleotide poly[d(G-z5C)]. Whereas the triphosphate decomposes in solution, the azacytosine analogue incorporated into DNA is stable under conditions preserving the double-helical structure. Poly[d(G-z5C)] undergoes the transition to the left-handed Z conformation at salt (NaCl and MgCl2) concentrations approximately 30% higher than those required for unsubstituted poly[d(G-C)]. However, the incorporation of azacytidine potentiates the formation at room temperature of the Z helix stabilized by the transition metal Mn2+; in the case of poly[d(G-C)], a heating step is required. The spectral properties of the two polymers in the B and Z forms are similar. Both left-handed forms are recognized by anti-Z DNA immunoglobulins, indicating that the DNAs bear common antigenic features. Poly[d(G-z5C)] is not a substrate for the DNA cytosine 5-methyltransferase from human placenta. It is a potent inhibitor of the enzyme when tested in a competitive binding assay. These results are compatible with a very strong, possibly covalent, mode of interaction between methyltransferases and DNA containing 5-azacytosine.     

Conformation of DNA in chromatin reconstituted from poly [d(A-T)] and the core histones.

Present results provide direct evidence of the nature of a conformational change in DNA when nucleosomes are formed from core histones and poly [d(A-T)]. First, we have found some features which have characteristic aspects of the A like conformation of DNA. Thus, an increased contribution due to a sugar conformation close to C3'-endo puckering is detected in the Raman spectra. In addition, the circular dichroism (C.D.) spectra of reconstituted chromatin with poly [d(A-T)] exhibits an increases intensity at about 262 nm. A second feature acquired by poly [d(A-T)] in nucleosome formation from core histones is related to the presence of a negative band at about 280 nm in the C.D.spectra. The nature of this change is correlated with a DNA conformation characterized by a decreased number of base pairs per turn (28,29). This indicates that these two features of reconstituted nucleosomes reflect the presence of two types of DNA conformations, which overall form is of the B type (22,36).     

Analysis of the 5'-termini of nascent DNA chains synthesized in permeable cells of Bacillus subtilis

The nascent DNA synthesized by permeable cells of Bacillus subtilis in the presence of 5'-mercurideoxycytidine triphosphate and 2',3'-dideoxyATP has been isolated and characterized. The newly synthesized DNA was isolated free from other cellular nucleic acids by affinity chromatography on thiol-substituted agarose. The number average chain length of the nascent DNA synthesized in one minute at 25 degrees C was 33 nucleotide residues, due to the chain-terminating action of 2',3'-dideoxyATP. Several lines of evidence indicated that at least 90% of the DNA thus isolated carried a terminally phosphorylated RNA moiety at its 5'-end: (1) the nascent DNA was resistant to exonucleolytic degradation by spleen phosphodiesterase unless first hydrolyzed by strong alkali or ribonuclease; (2) the 5'-termini of nascent DNA could not be phosphorylated by polynucleotide kinase unless first treated with alkaline phosphatase or subjected to hydrolysis by strong alkali or ribonuclease; (3) alkaline hydrolysis of nascent DNA labeled with 32P at the 5'-end released unlabeled DNA with a free 5'-terminus and 32P-labeled ribonucleoside 3',5'-bisphosphates; (4) ribonuclease degradation of similarly labeled material produced an unlabeled DNA-containing polynucleotide fraction and 32P-labeled ribo-oligonucleotides; (5) chromatography on dihydroxyboryl cellulose showed that the RNA moiety lacked a 3'-terminal cis-diol grouping (even after treatment with alkaline phosphatase) unless first subjected to the 3'-exonucleolytic action of bacteriophage T4 DNA polymerase. The sequence of the ribonucleotide chains was elucidated by end-group labeling with polynucleotide kinase and digestion with various ribonucleases. The ribonucleotide moiety was primarily three and four residues in length with the predominant sequence (pp)pApG(pC)1-2pDNA. The possibility that it represents a primer for discontinuous DNA synthesis is discussed.