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Biomedical subjects

R Lavery

Publications and source records attributed to R Lavery.

14 recordsLinked to original sources
PubMed1992

A comprehensive classification of nucleic acid structural families based on strand direction and base pairing.

We propose a classification of DNA structures formed from 1 to 4 strands, based only on relative strand directions, base to strand orientation and base pairing geometries. This classification and its associated notation enable all nucleic acids to be grouped into structural families and bring to light possible structures which have not yet been observed experimentally. It also helps in understanding transitions between families and can assist in the design of multistrand structures.

Base Composition↗
PubMed1992

The fine structure of two DNA dodecamers containing the cAMP responsive element sequence and its inverse. Nuclear magnetic resonance and molecular simulation studies.

1H and 31P n.m.r. (nuclear magnetic resonance) spectroscopy have been used in conjunction with molecular simulation to determine the structure of two DNA dodecamers. The first of these, CATGACGTCATG, contains the octameric sequence CRE (cAMP responsive element), while the second is the reversed sequence, GTACTGCAGTAC. Structure determination was based on both NOESY (nuclear Overhauser spectroscopy) derived distances and COSY (correlated spectroscopy) dihedral angle data. Access to the 31P spectra also allowed the epsilon backbone angles to be determined. Considerable care was taken in deriving structural parameters from the n.m.r. data and an excellent level of agreement is obtained with the simulated conformations. Both dodecamers are found to belong to the B-DNA family; however, there is a striking difference between the CRE sequence and its inverse, the former conformation alone showing a strong structural heterogeneity.

Base Sequence↗
PubMed1992

Conformational sub-states in B-DNA.

Theoretical studies of the sequence-dependent conformation of B-DNA have been carried out using Jumna, a helicoidal co-ordinate minimization algorithm. The results obtained for a series of six oligomers with repetitive sequences show that, with the exception of the homopolymers (dA)n.(dT)n and (dG)n.(dC)n, all sequences can adopt a variety of conformations characterized by considerable changes in helicoidal parameters and also in sugar puckers which adopt C(2')-endo (falling into 2 classes) or, in the case of pyrimidine nucleotides, O(1')-endo forms. These studies lead to an improved understanding of the role of base sequence on DNA conformation and point to a number of interesting correlations between the various structural parameters describing the double helix.

Algorithms↗
PubMed1992

DNA flexibility as a function of allomorphic conformation and of base sequence.

Systematic theoretical modeling of symmetric DNA oligomers, carried out earlier for the B conformation, is now extended to A-DNA. In contrast to the previous results, it is found that A-DNA shows no multiplicity of low-energy substate conformations. The possibilities of the Jumna algorithm are subsequently applied to studying deformations of the oligomers. Controlled winding and stretching deformations are used to study how the two allomorphs and different base sequences absorb such external stress. The results help explain the internal mechanics of the DNA double helix and the extent to which fine structure influences this behavior. The results point to some differences between the A and B double helices, but also to many similarities. Sequence effects on flexibility are relatively limited compared to their impact on optimal energy conformations. It is also shown that the conformational substates detected for B-DNA oligomers are preserved under deformation, but have little influence on its energetics.

Base Sequence↗
PubMed1992

Strand orientation of [alpha]-oligodeoxynucleotides in triple helix structures: dependence on nucleotide sequence.

The aims of the present theoretical study of the conformations of [alpha]-oligodeoxynucleotides forming triple helices with DNA duplexes are to understand the structural and energetic factors involved in [alpha]-triple helix formation by means of energy minimization, and to explain the experimentally observed dependence of strand orientation on the nucleotide sequence. It is found that the energetically preferred orientation of the [alpha]-oligonucleotide with respect to the homopurine strand depends on the sequence of the homopurine.homopyrimidine tracts. This is a consequence of the structural heteromorphism of base triplets in the intrinsically more stable reverse Hoogsteen hydrogen bonding configuration. Practical rules are proposed for determining the orientation of the nuclease-resistant [alpha]-oligodeoxynucleotide strand which will form the most stable triple helix.

Base Sequence↗
PubMed1992

A computational and experimental study of the bending induced at a double-triple helix junction.

We have studied the conformation of a 17 base-pair homopyrimidine.homopurine triple helix formed on a fragment of duplex DNA derived from Simian Virus SV40. Gel retardation assays indicate that an 80 base-pair fragment has an altered conformation when the triple helix is formed, which is most likely to result from an induced bend in the DNA. Investigation of the detailed conformation of the double helix-triple helix junctions has been performed by means of molecular modelling. Bending on the 5' and 3' sides of the third strand oligonucleotide are not located at equivalent positions with respect to the junctions, which is explained in terms of base stacking. The junction effects on DNA structure, induced by the requirement for cytosine protonation in the Hoogsteen-bonded strand to form CGC+ base triplets, are also discussed.

Base Sequence↗
PubMed1991

A new approach to the rapid determination of protein side chain conformations.

Two efficient algorithms have been developed which allow amino acid side chain conformations to be optimized rapidly for a given peptide backbone conformation. Both these approaches are based on the assumption that each side chain can be represented by a small number of rotameric states. These states have been obtained by a dynamic cluster analysis of a large data base of known crystallographic structures. Successful applications of these algorithms to the prediction of known protein conformations are presented.

Algorithms↗
PubMed1991

Triple helix structures: sequence dependence, flexibility and mismatch effects.

By means of molecular modelling, electrostatic interactions are shown to play an important role in the sequence-dependent structure of triple helices formed by a homopyrimidine oligonucleotide bound to a homopurine. homopyrimidine sequence on DNA. This is caused by the presence of positive charges due to the protonation of cytosines in the Hoogsteen-bonded strand, required in order to form C.GxC+ triplets. Energetic and conformational characteristics of triple helices with different sequences are analyzed and discussed. The effects of duplex mismatches on the triple helix stability are investigated via thermal dissociation using UV absorption.

Base Sequence↗
PubMed1991

Theoretical study of ethidium intercalation in triple-stranded DNA and at triplex-duplex junctions.

The contribution of different factors in the interaction of ethidium intercalated into various sequences of a triple helix, or in the region of the junction between the double- and triple-stranded DNA has been studied by energy minimization. It is found that in the total energy of the ethidium- triple helix complexes, a particular electrostatic contribution emerges due to the presence of protonated cytosines in the triple helix. This parameters is determinant in the sequence-specificity of ethidium binding to the triple helix. The preferred intercalation sites of ethidium in the triple helix are proposed. The interaction of ethidium at the triplex-duplex junction, and its effects are also discussed. This study is aimed at searching for new drugs specific for the triple helix, or for the triplex-duplex junctions.

Base Sequence↗
PubMed1990

Conformational and helicoidal analysis of the molecular dynamics of proteins: "curves," dials and windows for a 50 psec dynamic trajectory of BPTI.

A new procedure for the graphic analysis of molecular dynamics (MD) simulations on proteins is introduced, in which comprehensive visualization of results and pattern recognition is greatly facilitated. The method involves determining the conformational and helicoidal parameters for each structure entering the analysis via the method "Curves," developed for proteins by Sklenar, Etchebest, and Lavery (Proteins: Structure, Function Genet. 6:46-60, 1989) followed by a novel computer graphic display of the results. The graphic display is organized systematically using conformation wheels ("dials") for each torsional parameter and "windows" on the range values assumed by the linear and angular helicoidal parameters, and is present in a form isomorphous with the primary structure per se. The complete time evolution of dynamic structure can then be depicted in a set of four composite figures. Dynamic aspects of secondary and tertiary structure are also provided. The procedure is illustrated with an analysis of a 50 psec in vacuo simulation on the 58 residue protein, bovine pancreatic trypsin inhibitor (BPTI), in the vicinity of the local minimum on the energy surface corresponding to a high resolution crystal structure. The time evolution of 272 conformational and 788 helicoidal parameters for BPTI is analyzed. A number of interesting features can be discerned in the analysis, including the dynamic range of conformational and helicoidal motions, the dynamic extent of 2 degrees structure motifs, and the calculated fluctuations in the helix axis. This approach is expected to be useful for a critical analysis of the effects of various assumptions about force field parameters, truncation of potentials, solvation, and electrostatic effects, and can thus contribute to the development of more reliable simulation protocols for proteins. Extensions of the analysis to present differential changes in conformational and helicoidal parameters is expected to be valuable in MD studies of protein complexes with substrates, inhibitors, and effectors and in determining the nature of structural changes in protein-protein interactions.

Biological Evolution↗
PubMed1990

Base pair opening pathways in B-DNA.

Molecular modeling is used to study the opening pathways of bases within a B-DNA oligomer. It is demonstrated that many open states are possible for a single base pair, although a preference for opening towards the major groove of the double helix is found. In addition we show that opening is strongly influenced by the nature of the base involved and is also coupled in many cases to DNA bending.

Base Composition↗
PubMed1988

Sequence-targeted cleavage of nucleic acids by oligo-alpha-thymidylate-phenanthroline conjugates: parallel and antiparallel double helices are formed with DNA and RNA, respectively.

Oligodeoxynucleotides can be synthesized by using the alpha anomers of nucleoside units. Oligo-alpha-deoxynucleotides are resistant to nucleases and could be used to regulate gene expression in vivo. Theoretical calculations were carried out to determine the conformational energy of an oligomeric alpha-beta duplex (dA)5.(dT)5 where the adenosine strand contains natural beta-deoxyribonucleotides and the thymidine strand contains synthetic alpha-deoxyribonucleotides. These calculations predict that in the more stable B-like conformation the two strands of the double helix should run parallel to each other whereas in the more stable A-like conformation the two strands should adopt an antiparallel orientation. In order to test these predictions 1,10-phenanthroline was covalently attached to the 5'-end of an alpha-octathymidylate. In the presence of copper ions and a reducing agent (beta-mercaptopropionic acid), the (phenanthroline)2-copper complex generates OH. radicals that cleave phosphodiester bonds in the complementary sequence to which the alpha-octathymidylate is bound. By use of a 27mer oligo-beta-deoxynucleotide containing an octadeoxyadenylate sequence as a target for the phenanthroline-substituted alpha-(dT)8, cleavage was observed on the 5'-side of the (dA)8 sequence, demonstrating that the alpha-beta DNA-DNA hybrid formed a double helix with parallel orientation of the two strands. The same result was obtained when alpha-(dT)8 was bound to beta-(dA)n with n = 8 or 10. When a beta-oligoriboadenylate was used as a target, cleavage occurred exclusively on the 3'-side of the (rA)8 or (rA)10 sequence, indicating that the alpha-beta DNA-RNA hybrid formed a double helix with an antiparallel orientation of the two strands. When a phenanthroline-substituted beta-octathymidylate was used instead of the alpha-octathymidylate, an antiparallel double helix was formed independently of whether the target beta sequence was a DNA or an RNA.

Base Sequence↗
PubMed1988

The definition of generalized helicoidal parameters and of axis curvature for irregular nucleic acids.

An algorithm is presented which solves the problem of obtaining a rigorous helicoidal description of an irregular nucleic acid segment. Central to this approach is the definition of a function describing simultaneously the curvature of the nucleic acid segment in question and the corresponding stepwise variation of helicoidal parameters along the segment. Minimisation of this function leads to an optimal distribution of the conformational irregularity of the segment between these two components. Further, it is shown that this approach can be applied equally easily to single or double stranded nucleic acids. The results of this analysis yield both the absolute helicoidal parameters of individual bases/base pairs and the relative helicoidal parameters between successive bases/base pairs as well as the overall locus of the helical axis. The possibilities of this mathematical approach are demonstrated with the help of a computer program termed "Curves" which is applied to the study of a number of different nucleic acid structures.

Algorithms↗