Plasmids, Transposons, and Horizontal
Gene Transfer in Bacteria
Plasmids are circular, double-stranded DNA molecules, independent of the chromosome, found in many bacterial strains. They have never been detected in eukaryotes, except in some fungi. Their sizes range from very small (about 2 kbp) to very large (up to 400 kbp), one-tenth the size of a [typical] bacterial chromosome. They code for functions that are not normally necessary for the survival of the bacteria harboring them. They code for accessory functions, useful or even essential in some cases, such as, for example, antibiotic resistance, heavy metal resistance, fixation of atmospheric nitrogen, and, of course, tumorigenesis in plants.
Many types of plasmids are promiscuous, meaning that they can be exchanged between closely and even distandy related bacterial species. These exchanges occur naturally in, for example, hospital settings (where human pathogens can acquire antibiotic resistance), sewers, and soil. Bacteria use three modes of DNA acquisition or exchange: conjugation, transformation, and transduction. Transduction consists of bacteriophagemediated gene transfer but does not normally apply to plasmids. Plasmids can be acquired by transformation, a process of uptake of DNA from the environment by live bacteria after it has been released from dead, lysed bacteria. DNA can in some instances be released into the environment by living bacteria; for example, Thermoactinomyces vulgaris spontaneously releases DNA when its spores germinate.
Conjugation is a much more elaborate mechanism that bears a resemblance to sexual exchange in eukaryotes. Here, a [male] bacterial cell establishes a cytoplasmic bridge with a [female] via the formation of a conjugation tube. Plasmid DNA, in single-stranded form, can then travel through the