Although most plasmids are circular molecules of DNA there are occasional exceptions. Linear plasmids of double-stranded DNA have been found in a variety of bacteria and in fungi and higher plants. The best-characterized linear plasmids are found in those few bacteria such as Borrelia and Streptomyces that also contain linear chromosomes (see Ch. 5). Linear DNA replicons in bacteria are not protected by incompatibility The inability of two plasmids of the same family to co-exist in the same host cell mobilizability Ability of a non-transferable plasmid to be moved from one host cell to another by a transferable plasmid transferability Ability of a plasmid to move itself from one host cell to another
Occasional Plasmids are Linear or Made of RNA 429
Plasmids with different origins of replication and different replication genes are able to inhabit the same bacterial cell and are considered compatible (left). During cell division, both types of plasmid replicate; therefore, each daughter cell will inherit both plasmids, just like the mother cell. On the other hand, if two plasmids have identical origins and replication genes they are incompatible will not be replicated during cell division (right). Instead, the two plasmids are partitioned into different daughter cells. [Bacterial cells also contain circular chromosomes that divide in synchrony with cell division; however these have been omitted from this figure.]
Different replication systems
Different replication systems
Identical origin and replication genes cro
Cell division o o o o o o
A) BORRELIA hairpin/loop ends B) STREPTOMYCES tennis racquet ends
Linear plasmids have special structures to protect the ends of the DNA.
telomeres like the linear chromosomes of eukaryotes. Instead a variety of individual adaptations protect the ends from endonucleases.
In Borrelia there are not actually any free DNA ends. Instead hairpin sequences of single-stranded DNA form loops at the ends of both linear plasmids and chromosomes (Fig. 16.04A). Some animal viruses, such as the iridovirus that causes African swine fever, have similar structures. Different species of Borrelia cause Lyme's disease and relapsing fever. Their linear plasmids appear to encode both hemolysins that damage blood cells and surface proteins that protect the bacteria from the host immune system. Thus, as is true of many other infectious bacteria, the virulence factors of Borrelia are also largely plasmid borne.
The linear plasmids of Streptomyces are indeed genuine linear DNA molecules with free ends. They have inverted repeats at the ends of the DNA that are held together by proteins. In addition, special protective proteins are covalently attached to the 5'-ends of the DNA. The net result is a tennis racket structure (Fig. 16.04B). The DNA of adenovirus, most linear eukaryotic plasmids and some bacterial viruses show similar structures.
Linear plasmids are also found among eukaryotes. The fungus Flammulina velu-tipes, commonly known as the enoki mushroom, has two very small linear plasmids within its mitochondria. The dairy yeast, Kluyveromyces lactis, has a linear plasmid that normally replicates in the cytoplasm. However, on occasion the plasmid relocates to the nucleus where it replicates as a circle. Circularization is due to site specific recombination involving the inverted repeats at the ends of the linear form of the plasmid. The physiological role of these plasmids is obscure.
RNA plasmids are rare and most are poorly characterized. Examples are known from plants, fungi and even animals. Some strains of the yeast, Saccharomyces cere-visiae, contain linear RNA plasmids. Similar RNA plasmids are found in the mitochondria of some varieties of maize plants. RNA plasmids are found as both single-stranded and double-stranded forms and replicate in a manner similar to certain RNA viruses. The RNA plasmid encodes RNA-dependent RNA polymerase that directs its own synthesis. Unlike RNA viruses, RNA plasmids do not contain genes for coat proteins. Sequence comparisons suggest that these RNA plasmids may have evolved from RNA viruses that have taken up permanent residence after losing the ability to move from cell to cell as virus particles.
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