The following points highlight the nine main types of plasmids in bacterial cytoplasm. The types are: 1. Sex Factor 2. R (resistance) Plasmids 3. Heavy-Metal Resistance Plasmids 4. Col Plasmids 5. Degradative Plasmids 6. Penicillinase Plasmid of Staphylococcus Aureus 7. Cryptic Plasmids 8. Ti-Plasmids of Agrobacterium Tumifaciens 9. Ri-Plasmids.
Type # 1. Sex Factor or Fertility (F) Factor:
The plasmids of male cells that confer on their host cells the capability to transmit chromosomal markers but not the other properties are called sex factor or F factor (Fig.4.22C). For the first time the F factor was discovered in E. coli.
The term sex factor is used in two ways:
(a) As genetic name for all plasmids which determines host conjugation and their own transfer irrespective of the transfer of bacterial chromosome, and
(b) To describe the set of genes on plasmids whose products mediate the conjugation process.
Sometimes the F factor transfers a non-conjugative plasmid also which is present with it in a cell. A new strain was isolated from F+ cultures that underwent sexual recombination with F– cells. This new strain had recombinant rate about 103 times greater than F+ × F– cells. These strains were called high frequency recombination (Hfr) strains. The Hfr strains contained both bacterial PNA and plasmid DNA.
Type # 2. R (Resistance) Plasmids:
During 1946 several out breaks of dysentery by Shigella occurred in Japan and resistance against antibiotic drugs developed. In 1955, in Japan a strain of S. dysenteriae causing dysentery was isolated that was resistant to four drugs viz., sulfanilamide, streptomycin, chloramphenicol and tetracycline.
By 1964, over 50% all Shigella hospital isolates in Japan became multiple drugs resistant. Similar strains were also isolated from hospital in London in 1962, and that of Salmonella in England in 1965.
K. Ochiai and co-workers in Japan found that the genetic elements governing the drug resistance are transferable to the other bacterial cell through the process of conjugation. Further studies explained that the resistance genes are associated in various combinations as parts of transmissible plasmids known as R factor.
Throughout the world R factors spread among populations of enteric bacteria due to rapid use of antibiotics. Different R factors having different sets of resistance genes (ranging from one to eight) are required from different chemicals. Therefore, there are several genes of R factor which confer resistance against antibodies.
An R factor consists of two segments of DNA, one the resistance transfer factor (RTF) and the second resistant determinants (r-determinants). The RTF contains genes for replication and transmission of plasmid, and the r-determinants are on the another segment. Some of r-determinants replicate autonomously.
The G+C content, molecular weight and buoyant density of these two segments vary. When two bacteria (one containing R plasmid and the other devoid of R) conjugate the R plasmid is transferred to the later that lacks R plasmid.
Type # 3. Heavy-Metal Resistance Plasmids:
There are several bacterial strains that contain genetic determinant of resistance to heavy metals viz., Hg++, Ag+, Cd++, Co++, CrO4, Cu++, Ni++, P+++, Zn++, etc.. These determinants for resistance are often found on plasmids and transposons.
In the 1970s in Tokyo both heavy metal resistance and antibiotic resistance were found with high frequency in E. coli isolated from hospital patients, whereas heavy metal resistance plasmids without antibiotic resistance determinants were found in E. coli from an industrially polluted river.
Bacteria that have been found resistant to heavy metals are E. coli and Staphylococcus aureus (As), Pseudomonas aeruginosa, P. fluorescens, E. coli (Cr), B. subtilis, Alcaligens eutrophus (Cd), Shigella spp, E. coli (fig), Pseudomonas syringae.
Type # 4. Col Plasmids:
There are many bacterial strains that produce proteinaceous toxins known as bacteriocin which are lethal to other strains of the same genus. Toxins secreted by the strains of E. coli are called colicins.
It kills the sensitive cells. Synthesis of colicins is specified by the plasmids present in E. coli cells but not by bacterial Chromosome. These plasmids associated with colicin production are called colicinogeny (Col) factor. There are several Col plasmids such as Col B, Col E, Col I, and Col V which produce different types of colicins.
Some Col plasmids carry fertility determinants i.e. a set of genes governing conjugation and transfer of plasmids (e.g. Col B, Col V). These can be called conjugative plasmids. The second type is the non-conjugative plasmid which is non-transmissible by their own means (e.g. col E). However, when a cell contains both the plasmids, it transfers both.
Type # 5. Degradative Plasmids:
Much work has been done on degradative plasmid of Pseudomonas. The pseudomonads have been found to catalyse a number of unusual complex organic compounds through the special metabolic pathways.
Anand Mohan Chakrabarty, an India-borne American scientist, has isolated plasmids from a number of cultures of Pseudomonas putida which could utilize a number of complex organic chemicals such as 2, 4-D salicylate, 3-chloro-benzene, biphenyls, etc..
Special genes present on different plasmids confer degradation capacity to the species of Pseudomonas. For example, the camphor (CAM) plasmid of P.putida encodes enzyme for degradation of camphor, octane (OCT) plasmid degrades octane, XYL plasmid degrades xylene and toluene, NAH plasmid degrades naphthalene, SAL degrades salicylate, etc.
These plasmids are transmissible between the strains of species of P.putida through conjugation. More interestingly A.M. Chakrabarty succeeded in transferring the four plasmids, OCT, XYL, CAM and NAH present in four different strains of P.putida into one strain and called it superbug (oil eating bug) (Fig. 4.23). The superbug can degrade the above four types of substrate for which four types of plasmids are recognised.
Type # 6. Penicillinase Plasmid of Staphylococcus Aureus:
Staphylococcus aureus is a Gram- positive bacterial pathogen causing infection of skin and wounds of humans. After treatment with penicillin antibiotic, several penicillin-resistant staphylococci developed by 1950 through out the world.
High level resistance to penicillin was possible due to secretion of an enzyme, penicillinase which degrades penicillin by hydrolyzing its β-lactum ring. During 1970s R.P. Novick isolated the genes (Pt) from plasmid encoding penicillinase.
There is a large variety of penicillinase plasmids designated as α, β, Ƴ, etc. on the basis of markers present on them and production of chemically different penicillinases. Penicillinase plasmids also confer resistance against kanamycin, neomycin, tetracycline, streptomycin and chloramphenicol. Molecular weight of Kanr and Neor plasmids has been found to about 15 × 106 Daltons and that of Tetr, Strr and Chlr as 3 × 106 Dalton.
Penicillinase plasmids do not promote conjugation but these can be transferred from cell to cell by phage transduction at a rate that could be sufficient to balance the spontaneous loss in natural population of staphylococci.
Type # 7. Cryptic Plasmids:
During isolation of plasmid DNA from a large number of bacteria, it was found that every bacterium contained a low molecular weight DNA as plasmid. They do not carry any gene, therefore, they are non-functional. These plasmids have been called as cryptic plasmid. It seems that the presence of plasmids is a rule rather than exceptions.
Type # 8. Ti-Plasmids of Agrobacterium Tumifaciens:
A. tumifaciens is a Gram-negative soil bacterium that infects over 300 dicots and causes crown gall disease at collar region. It produces tumours, therefore, it is oncogenic. The Ti- refers to tumour inducing plasmids. The size of Ti plasmid ranges from 180-205 Kb. It consists of T-DNA (transfer DNA) of about 20 Kb which comprises of two adjacent independently encoding DNA segments, the right T-DNA (TR-DNA) and the left T-DNA (TL-DNA).
T-DNA encodes enzymes for the synthesis of auxin and cytosine which interfere plant metabolism, and develop tumour and enable the infected plant to produce a nitrogenous compound called Opines. Opines are metabolized by the bacterium as a source of carbon and nitrogen.
In addition to T-DNA, the Ti plasmid also consists of several genes such as vir (for virulence), ori (for origin of replication), tra (for transfer), noc (for nopaline catabolism in nopaline plasmid), arc (for arginine catabolism), and occ (for octopine catabolism in octopine plasmid) genes.
The Ti-plasmid is divided into the four groups: octopine Ti- plasmids (e.g. pTiB6, pTiAG), nopaline Ti-plasmid (e.g. pTiT37, pTiC58), leucinopine plasmids and succinamopine plasmids. The first two plasmids octopine and nopaline plasmids have been extensively studied. Diagram of nopaline plasmid is shown in Fig. 4.24A.
Type # 9. Ri-Plasmids:
The Ri (root inducing) plasmids are found in Agrobacterium rhizogenes which cause hairy root ditease in plants. The Ri-plasmids are closely related to Ti plasmids. The size of Ri-plasmids varies from 190-240 Kb; thus they are large sized plasmids.
On the basis of opine production in the infected plants the Ri plasmids are put into three groups: manopine Ri-plasmids (e.g. pRiTR7, pRi8196), agropine plasmids (e.g. pRiA4, pRil855) and cucumopine plasmids (e.g.pRi2659). Diagram of an agropine Ri-plasmids is given in Fig.4.24 B. The mechanism of T-DNA transfer and tumour morphology has been described in detail in A Text Book of Bio-technology by R.C. Dubey (2006).