Chromosomal DNA

Cell-to-cell contact required

Small fraction of chromosome



F plasmid

Cell-to-cell contact required

Entire F plasmid



210 Chapter 8 Bacterial Genetics world, being found in most members of the Bacteria and the Archaea as well as in eukaryotic algae, fungi, and protozoa. They play key roles in the lives of these organisms with their function in the life of bacteria being especially well understood. One of the most interesting plasmids is the tumor-inducing (Ti) plasmid of Agrobacterium. It allows Agrobacterium to conjugate with plants (see Perspective 8.2) ■ plasmid, p. 66

A study of a wide variety of plasmids in many different bacteria has led to the following generalizations:

■ Plasmids usually are covalently closed circular double-stranded DNA molecules.

■ Most prokaryotes contain one or more different plasmids; their functions, however, are often unknown.

■ The traits coded by plasmids generally provide the bacterial cell with useful but not indispensable capabilities. For example, the enzymes of the glycolytic pathway are never encoded on plasmids, since they are indispensable to the cell. Many members of the genus Pseudomonas, however, can grow not only on such common sugars as glucose, but also on unusual compounds, such as camphor, if they have a certain plasmid. Thus, the plasmid extends the cell's biochemical capabilities for degrading certain compounds. Plasmids code for a wide variety of other traits, some of which are listed in table 8.4.

■ Plasmids vary in size from a few genes to several hundred. The only function that all plasmids must carry out is replication and so all of them are replicons, pieces of DNA that have the genetic information required to replicate. The smallest plasmids contain only the information necessary for replication. The larger ones code for more functions, such as antibiotic resistance.

■ The number of copies per cell of each kind of plasmid varies, depending on the plasmid. For example, the F

Table 8.4 Some Plasmid-Coded Traits


Organisms in Which Trait Is Found

Antibiotic resistance

Escherichia coli, Salmonella sp., Neisseria sp., Staphylococcus sp., Shigella sp., and many other organisms

Pilus synthesis

E. coli, Pseudomonas sp.

Tumor formation in plants

Agrobacterium tumefaciens

Nitrogen fixation (in plants)

Rhizobium sp.

Oil degradation

Pseudomonas sp.

Gas vacuole production

Halobacterium sp.

Insect toxin synthesis

Bacillus thuringiensis

Plant hormone synthesis

Pseudomonas sp.

Antibiotic synthesis

Streptomyces sp.

Increased virulence

Yersinia enterocolitica

Toxin production

Bacillus anthracis

plasmid is present in only one to two copies per cell and is called a low-copy-number plasmid. Other plasmids, called high-copy-number plasmids, may be present in more than 500 copies per cell.

■ Although plasmids can be transferred to other bacteria by the three methods we have discussed, conjugation is by far the most important means of transfer for many Gram-negative species of bacteria.

■ Most plasmids can multiply only in one species of bacteria. They are termed narrow host range plasmids. The F plasmid is an example of such a plasmid. A few, however, termed wide host range plasmids, can multiply in many different species of bacteria.

R Plasmids

Among the best studied and most important groups of plas-mids are the resistance or R plasmids. They are so named because they confer resistance to many different antimicrobial medications and heavy metals, such as mercury and arsenic. Like antimicrobial medications, heavy metals are found in the hospital environment, and it is in this environment that bacteria containing R plasmids are commonly found. Many of these plasmids are composed of two parts: the resistance genes or R genes, which code for the resistance traits, and the resistance transfer factor or RTF, which codes for the transfer of the plasmid to other bacteria by conjugation (figure 8.22). In a self-transmissible plasmid, the RTF portion contains genes that code for pilus synthesis, the origin of transfer, and mobilization genes, all of which are required for plasmid transfer.

The R genes code for resistance to widely used antimicrobial medications, which can include sulfanilamide, strepto-

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