Plasmids

In addition to having a chromosome, many bacteria possess plasmids, small, circular DNA molecules ( FIGURE 8.6). Some plasmids are present in many copies per cell, whereas others are present in only one or two copies. In general, plas-mids carry genes that are not essential to bacterial function but that may play an important role in the life cycle and growth of their bacterial hosts. Some plasmids promote mating between bacteria; others contain genes that kill other bacteria. Of great importance, plasmids are used extensively in genetic engineering (Chapter 18) and some of them play a role in the spread of antibiotic resistance among bacteria.

Most plasmids are circular and several thousand base pairs in length, although plasmids consisting of several hundred thousand base pairs also have been found. Possessing its own origin of replication, a plasmid replicates independently of the bacterial chromosome. Replication proceeds from the origin in one or two directions until the entire plasmid is copied. In I FIGURE 8.7, the origin of replication is oriV. A few plasmids have multiple replication origins.

Mammalian Cell Antibiotic Kill Curve

4 8.6 Neisseria gonorrhoeae (a bacterium that causes gonorrhea), like many other bacteria, contains plasmids in addition to its chromosome.

The connected plasmids (indicated by the arrow) have just replicated. (A. B. Dowsett/Science Photo Library/ Photo Researchers.)

Bacterial Chromosome Replication Diagram

I 8.7 A plasmid replicates independently of its bacterial chromosome. Replication begins at the origin of replication (oriV) and continues around the circle. In this diagram, replication is taking place in both directions; in some plasmids, replication is ione direction only. (Photo from Photo Researchers.)

Sequences that regulate insertion into the bacterial chromosome: IS2-

Genes that regulate plasmid transfer to other cells

Sequences that regulate insertion into the bacterial chromosome: IS2-

Genes that control plasmid replication:

oriV (origin of replication)

inc rep

Origin of transfer

F factor

Origin of transfer

F factor

Genes that control plasmid replication:

oriV (origin of replication)

inc rep

A 8.8 The F factor, a circular episome of E. coli, contains a number of genes that regulate transfer into the bacterial cell and insertion into the bacterial chromosome. It contains a number of genes that regulate its transfer to other cells and that control replication. Replication is initiated at oriV. Insertion sequences (Chapter 11) IS3 and IS2 control insertion into the bacterial chromosome and excision from it.

Episomes are plasmids that are capable of either freely replicating or integrating into the bacterial chromosomes. The F (fertility) factor of E. coli ( FIGURE 8.8) is an epi-some that controls mating and gene exchange between E. coli cells, as will be discussed in the next section.

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