Spermidine-condensed phi X174 DNA cleavage by micrococcal nuclease: torus cleavage model and evidence for unidirectional circumferential DNA wrapping
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Abstract:
Spermidine-condensed phi X174 replicative form (RF) II DNA was digested with micrococcal nuclease to yield seven identifiable DNA bands forming an arithmetic fragment-length series. The DNA monomer unit length was found to be 780 +/- 80 base pairs. This result is most consistent with a proposed model for micrococcal nuclease cleavage of a DNA torus organized by the unidirectional, circumferential wrapping of B-geometry DNA. By a topological consideration, the blunt-end-rod-fusion model for torus formation [Eickbush, T. H. & Moudrianakis, E. N. (1978) Cell 13, 295-306] is shown to be inconsistent with our empirical solution results. We propose a continuous, circumferential DNA wrapping model in which a significant fraction of the collapsed circular phi X174 RFII DNA molecules form regular toruses comprised of seven complete, unidirectional double-helical wraps.
Insights
Micrococcal nuclease digestion of spermidine-condensed phi X174 replicative form II DNA revealed seven fragments. This supports a model of DNA toruses formed by continuous, unidirectional double-helical wraps.
Area of Science:
- Molecular Biology
- Biophysics
- Structural Biology
Background:
- Phi X174 replicative form II DNA condensation is crucial for packaging.
- Understanding DNA organization within condensed structures informs genome packaging mechanisms.
Purpose of the Study:
- To investigate the structural organization of spermidine-condensed phi X174 replicative form II DNA.
- To determine the DNA monomer unit length and cleavage pattern by micrococcal nuclease.
Main Methods:
- Digestion of spermidine-condensed phi X174 replicative form II DNA with micrococcal nuclease.
- Analysis of DNA fragment lengths using gel electrophoresis.
- Topological analysis to evaluate DNA torus models.
Main Results:
- Seven DNA bands were observed, forming an arithmetic fragment-length series.
- The DNA monomer unit length was determined to be 780 +/- 80 base pairs.
- A continuous, circumferential DNA wrapping model was supported, inconsistent with the blunt-end-rod-fusion model.
Conclusions:
- Phi X174 replicative form II DNA forms regular toruses composed of seven unidirectional double-helical wraps.
- The proposed model explains the observed DNA fragmentation pattern and unit length.
- This provides insights into DNA organization and packaging in viral systems.