Basic Plasmid Characteristics Size and Copy Number

Naturally occurring plasmids vary greatly in their physical properties, a few examples of which are shown in Table 1. They range in size from <2-kilobase pair (kbp) plas-mids, which can be considered to be elements simply capable of replication, to

From: Methods in Molecular Biology, Vol. 235: E. coli Plasmid Vectors Edited by: N. Casali and A. Preston © Humana Press Inc., Totowa, NJ

Table 1

Examples of Plasmids with Different Physical Characteristics

		Plasmid size	Plasmid	Plasmid copy
Plasmid	Host	(kbp)	geometry	number	Ref.
pUB110	Bacillus subtilis	2.3	Circular	20-50	7
ColEl	Escherichia coli	6.6	Circular	10-30	9
lp25	Borrelia burgdorferi	24.2	Linear	1-2	6
pNOB8	Sulfolobus sp.a	41.2	Circular	2-40	10
F	Escherichia coli	99.2	Circular	1-2	11
SCP1	Streptomyces coelicolor	350.0	Linear	4	12
pSymA	Sinorhizobium meliloti	1354.2	Circular	2-3	8

Fig. 1. Plasmid complement of a multiplasmid-containing strain of Lactococcus lactis ana­lyzed by agarose gel electrophoresis. The approximate sizes of the plasmids are indicated (kbp).

megaplasmids that are many hundreds of kilobase pairs in size. At the upper end of this scale, the distinction between a megaplasmid and a minichromosome can become obscure. Some bacterial species simultaneously harbor multiple different plasmids that can contribute significantly to the overall genome size of the host bacterium (see Fig. 1) (6,13). As an example, the symbiotic soil bacterium Sinorhizobium meliloti has three replicons (3.65, 1.68, and 1.35 megabase pairs [Mbp]) in addition to its chromosome (6.69 Mbp) (8). The smallest megaplasmid, pSymA, can be cured from the host bacte­rium under laboratory conditions but provides nodulation and nitrogen-fixation func­tions that are important for the symbiotic interaction of the bacterium and its plant host.

Different plasmids have different copy numbers per chromosome equivalent. Some plasmids have a steady-state copy number of one or a few copies, whereas other, mainly small, plasmids are present at tens or even hundreds of copies per chromosome. The plasmid copy number is determined by replication control circuits that are discussed under Subheading 4, and in detail by del Solar and Espinosa (14). Therefore, the con­tribution of plasmid DNA to the host bacterium's genome depends on the number of different plasmids that the bacterium harbors, as well as their size and copy number.