Molecular Cloning of DNA from the R-Plasmid R6K
Bacterial plasmids are non-essential in the sense that they may be lost from a cell under most circumstances without affecting cell viability. As adequately documented elsewhere in this volume, the presence of a plasmid may confer upon a host cell the capacity to survive in an adverse environment or to better compete with organisms of the same or related species. Plasmids have a range of molecular mass from 0.5 × 106 daltons to greater than 150 × 106 daltons and vary in their mol fraction guanine and cytocine content from 0.39 to 0.72. Consequently it is as difficult to make generalizations about the basic biology of plasmids as it is about the microorganisms in which their life cycle takes place. Nonetheless at the molecular level one can make the generalization that plasmids are double-stranded covalently closed molecules of DNA. In addition plasmids generally fall into two molecular classes. One class, best typified by ColEl, is of relatively small size (generally less than 10 × 106 daltons in mass), non-conjugative and is generally found as a multi-copy pool within its host cells. The other class, best typified by the classical sex factor, F, is of relatively large molecular mass (generally greater than 30 × 106 daltons), usually conjugative and present in a limited number of copies per host cell. One (but certainly not the only) exception to these general molecular properties of plasmids is R6K.
KeywordsEcoRI Site Replication Function Bacterial Plasmid Superhelical Density Superhelical Turn
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