Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Mobile Genetic Elements
Plasmids

Plasmids are extrachromosomal hereditary factors found in bacterial Cells. They are capable of transferring Genetic information from one bacterium to another. Some plasmids can reversibly integrate into the chromosome; these are known as episomes. Episomes typically contain IS or Tn elements that enable them to integrate into the chromosomal Structure.

Plasmid DNA accounts for 0.1 – 5 % of the chromosomal size. In most cases, plasmids are circular, covalently closed, supercoiled DNA molecules. However, linear plasmids also exist, in which case the ends are either protected by Proteins or covalently joined. Plasmids carry genes that are non-essential for bacterial survival, including: Antibiotic Resistance genes; genes conferring resistance to heavy metals and other toxic agents; toxin genes; genes enabling Bacteria to utilize unconventional substrates; and various other genes.

Plasmids rely on the cellular Replication machinery for their own replication. Notably, this process occurs independently of chromosomal replication. Each plasmid constitutes a replicon whose replication is strictly regulated, ensuring that each plasmid is maintained in the bacterial Cell at a characteristic copy number. Accordingly, plasmids are classified into:

a) single-copy plasmids, which are present at a single copy per cell;

b) multicopy plasmids, which are typically present in 10 – 20 copies per cell;

c) relaxed-control plasmids, which can accumulate in The Cell up to 1,000 copies.

Plasmids can be transmitted from one bacterium to another via conjugation, a property that defines transmissible plasmids. These plasmids contain tra genes encoding The formation of sex pili on the cell surface. Through these structures, a plasmid-bearing bacterial cell can attach to a plasmid-free bacterium and form a bridge, allowing plasmid DNA to be transferred into the recipient cell. In addition to transmissible plasmids, there are mobilizable plasmids. Although the latter lack tra genes, they can hijack the conjugation machinery of transmissible plasmids to move from one cell to another.

During The transfer of plasmid DNA between cells, a specialized protein introduces a nick into one of the DNA strands and covalently attaches to its 5’-end. This strand is then translocated through the conjugation bridge into the recipient cell, while the complementary strand remains in the donor. Subsequently, the cellular DNA Synthesis machinery uses the single-stranded DNA templates to synthesize complementary strands, and the DNA molecules in the recipient cells circularize. Consequently, conjugation results in one plasmid copy being retained in the donor cell while another is established in the recipient.

In some instances, conjugation is not required for plasmid transfer. Certain small plasmids can be transmitted between bacteria packaged within Bacteriophages. Furthermore, bacteria can take up free plasmid DNA directly from their environment, which typically originates from the lysis of dead bacterial cells.

Plasmids can exhibit a broad host range and are referred to as promiscuous (or cosmopolitan) plasmids. Some plasmids are capable of replicating in both Gram-positive and Gram-negative bacteria, whereas others can maintain themselves in both prokaryotic and Yeast cells.