Classification of plasmids

Plasmids, IS Elements, Transposons and Retroelements
Classification of plasmids
Replication, transfer and recombination in plasmids
Insertion sequences or IS elements
Transposons and controlling elements
Transposons in prokaryotes
Transposons in eukaryotes
Retrolelements (viral and non-viral)
Mechanism of Transposition
Uses of Transposons
There are mainly three types of plasmids, which will be described in this section : (i) F or sex factor, responsible for transfer of genetic material from one strain to another; (ii) R factor, responsible for drug resistance and (iii) Col factor, responsible for colicin production. Some plasmids may share the properties of more than one type of plasmids but they are still classified only in one of the three categories. For instance, some R plasmids control drug resistance as well as fertility, but they are never called F factor, but only R plasmids. There may be other plasmids, which have properties other than the three described above. For instance, some plasmids confer pathogenicity on host bacteria. This classification, therefore, has been considered unsatisfactory, but no other satisfactory classification has been evolved so far. The three types of plasmids, F, R and Col, will be briefly described in this section. Characteristics of some of these plasmids are given in Table 15.1.

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The F factor or sex factor. The F factor or sex factor has already been described in some detail in the Sexuality and Recombination in Bacteria and Viruses. It may be recalled that when F+ strain (carrying F factor) comes in contact with F-, two processes may follow, one involving transfer of F factor, so that F- becomes F+ and second involving transfer of a segment of bacterial chromosome, which is facilitated by insertion of F factor within the bacterial chromosome at one of the several specific sites available, leading to the production of Hfr (high frequency recombinant).

F factor has been included in the category of cell constituents called episomes, since F can exist in 'free' or 'integrated' state. However, as earlier pointed out, all plasmids are not episomes, since they are not always able to attach to the main chromosome. Nevertheless F is a plasmid, because it is a DNA molecule smaller than main chromosome and can be free and replicate independently of bacterial chromosome.

The R plasmid or resistance transfer factor (RTF). R plasmids or R factor, when present, confer on bacteria resistance to several antibiotics, such as streptomycin, tetracycline, chloramphenicol and sulphonamide. This was discovered in Japan in 1956, when a strain of bacteria called Shigella was shown to acquire resistance against several drugs in one step. This was shown to be due to transmission of resistance from one strain to another and was later attributed to R plasmid, which not only confers resistance, but also enables bacteria carrying them to conjugate (even in the absence of F factor) and transfer the factor to other bacteria, related or quite unrelated. The genes for drug resistance were later shown to be due to a transposon (Tn3) with terminal inverted repeat (IR) sequences (see later).

It has been shown, that like F factor, R is also a small extrachromosomal DNA circle, and is therefore a plasmid. Sometimes a plasmid may consist of two parts, one bearing genes for resistance (R factor) and the other responsible for transfer of plasmid (resistance transfer factor or RTF). In some cases like penicillin resistance in Staphylococcus, the resistance may not be associated with ability to transfer (RTF). In such a case transfer may be brought about by transduction.

The col factors. Colicins are toxic proteins, which are produced by bacteria (Escherichia, Shigella and Salmonella)and which kill bacteria other than those, which produce them. This property of bacteria is a genetic property and depends on elements called colicinogenic factors now popularly known as Col plasmids, which also fit into the definition of episomes. Col factors resemble phages, since they cause death of bacteria; but they differ from phages, since the death of bacteria is not accompanied with simultaneous release of Col factors as in the case of lysis due to phages.

Genetic studies suggest that colicin production is not determined by genes on bacterial chromosomes but by genes on Col plasmids, which can be transmitted from one bacterium to another. In these properties, Col plasmids resemble F factor or R factor. Like transfer of R factor, transfer of Col factor does not depend on F factor. Therefore, the transfer of Col can take place between two F- strains, one carrying Col and the other lacking it. There are however, some Col plasmids (e.g. Col E1)which require the aid of sex factor for its transfer. Like R factor, some Col factors also have the ability to mobilize transfer of bacterial chromosomes, but unlike R factor, such a transfer may or may not be due to insertion of Col factor as in F+ and Hfr strains (for F factor).