Bacteriophage T7 helicase/primase proteins form rings around single-stranded DNA that suggest a general structure for hexameric helicases. 1995

E H Egelman, and X Yu, and R Wild, and M M Hingorani, and S S Patel
Department of Cell Biology and Neuroanatomy, University of Minnesota Medical School, Minneapolis 55455, USA.

Most helicases studied to date have been characterized as oligomeric, but the relation between their structure and function has not been understood. The bacteriophage T7 gene 4 helicase/primase proteins act in T7 DNA replication. We have used electron microscopy, three-dimensional reconstruction, and protein crosslinking to demonstrate that both proteins form hexameric rings around single-stranded DNA. Each subunit has two lobes, so the hexamer appears to be two-tiered, with a small ring stacked on a large ring. The single-stranded DNA passes through the central hole of the hexamer, and the data exclude substantial wrapping of the DNA about or within the protein ring. Further, the hexamer binds DNA with a defined polarity as the smaller ring of the hexamer points toward the 5' end of the DNA. The similarity in three-dimensional structure of the T7 gene 4 proteins to that of the Escherichia coli RuvB helicase suggests that polar rings assembled around DNA may be a general feature of numerous hexameric helicases involved in DNA replication, transcription, recombination, and repair.

UI MeSH Term Description Entries
D008854 Microscopy, Electron Microscopy using an electron beam, instead of light, to visualize the sample, thereby allowing much greater magnification. The interactions of ELECTRONS with specimens are used to provide information about the fine structure of that specimen. In TRANSMISSION ELECTRON MICROSCOPY the reactions of the electrons that are transmitted through the specimen are imaged. In SCANNING ELECTRON MICROSCOPY an electron beam falls at a non-normal angle on the specimen and the image is derived from the reactions occurring above the plane of the specimen. Electron Microscopy
D008958 Models, Molecular Models used experimentally or theoretically to study molecular shape, electronic properties, or interactions; includes analogous molecules, computer-generated graphics, and mechanical structures. Molecular Models,Model, Molecular,Molecular Model
D004265 DNA Helicases Proteins that catalyze the unwinding of duplex DNA during replication by binding cooperatively to single-stranded regions of DNA or to short regions of duplex DNA that are undergoing transient opening. In addition, DNA helicases are DNA-dependent ATPases that harness the free energy of ATP hydrolysis to translocate DNA strands. ATP-Dependent DNA Helicase,DNA Helicase,DNA Unwinding Protein,DNA Unwinding Proteins,ATP-Dependent DNA Helicases,DNA Helicase A,DNA Helicase E,DNA Helicase II,DNA Helicase III,ATP Dependent DNA Helicase,ATP Dependent DNA Helicases,DNA Helicase, ATP-Dependent,DNA Helicases, ATP-Dependent,Helicase, ATP-Dependent DNA,Helicase, DNA,Helicases, ATP-Dependent DNA,Helicases, DNA,Protein, DNA Unwinding,Unwinding Protein, DNA,Unwinding Proteins, DNA
D004277 DNA, Single-Stranded A single chain of deoxyribonucleotides that occurs in some bacteria and viruses. It usually exists as a covalently closed circle. Single-Stranded DNA,DNA, Single Stranded,Single Stranded DNA
D001665 Binding Sites The parts of a macromolecule that directly participate in its specific combination with another molecule. Combining Site,Binding Site,Combining Sites,Site, Binding,Site, Combining,Sites, Binding,Sites, Combining
D012316 RNA Nucleotidyltransferases Enzymes that catalyze the template-directed incorporation of ribonucleotides into an RNA chain. EC 2.7.7.-. Nucleotidyltransferases, RNA
D016571 Neural Networks, Computer A computer architecture, implementable in either hardware or software, modeled after biological neural networks. Like the biological system in which the processing capability is a result of the interconnection strengths between arrays of nonlinear processing nodes, computerized neural networks, often called perceptrons or multilayer connectionist models, consist of neuron-like units. A homogeneous group of units makes up a layer. These networks are good at pattern recognition. They are adaptive, performing tasks by example, and thus are better for decision-making than are linear learning machines or cluster analysis. They do not require explicit programming. Computational Neural Networks,Connectionist Models,Models, Neural Network,Neural Network Models,Neural Networks (Computer),Perceptrons,Computational Neural Network,Computer Neural Network,Computer Neural Networks,Connectionist Model,Model, Connectionist,Model, Neural Network,Models, Connectionist,Network Model, Neural,Network Models, Neural,Network, Computational Neural,Network, Computer Neural,Network, Neural (Computer),Networks, Computational Neural,Networks, Computer Neural,Networks, Neural (Computer),Neural Network (Computer),Neural Network Model,Neural Network, Computational,Neural Network, Computer,Neural Networks, Computational,Perceptron
D017123 Bacteriophage T7 Virulent bacteriophage and type species of the genus T7-like phages, in the family PODOVIRIDAE, that infects E. coli. It consists of linear double-stranded DNA, terminally redundant, and non-permuted. Coliphage T7,Enterobacteria phage T7,Phage T7,T7 Phage,Phage, T7,Phages, T7,T7 Phages
D046911 Macromolecular Substances Compounds and molecular complexes that consist of very large numbers of atoms and are generally over 500 kDa in size. In biological systems macromolecular substances usually can be visualized using ELECTRON MICROSCOPY and are distinguished from ORGANELLES by the lack of a membrane structure. Macromolecular Complexes,Macromolecular Compounds,Macromolecular Compounds and Complexes,Complexes, Macromolecular,Compounds, Macromolecular,Substances, Macromolecular
D019915 DNA Primase A single-stranded DNA-dependent RNA polymerase that functions to initiate, or prime, DNA synthesis by synthesizing oligoribonucleotide primers. EC 2.7.7.-. Primase,Bacteriophage T7 Gene 4 Protein,DnaG (Primase),DnaG Gene Product,DnaG Protein,T7 DNA Primase-Helicase Protein,T7 DNA-Priming Protein,T7 gene-4 protein,DNA-Priming Protein, T7,Primase, DNA,Protein, T7 DNA-Priming,T7 DNA Primase Helicase Protein,T7 DNA Priming Protein,T7 gene 4 protein

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