Plasmid can integrate into chromosome through 2 mechanism. Plasmid DNA transfer involves replication by the rolling circle mechanism (Fig. Resistance to toxic heavy metals (such as cadmium, cobalt, silver, lead, and mercury) is also often encoded by plasmids. In assays designed to measure in vitro nicking of RecBCD at its hot spot sequence Chi (3′GCTGGTGG5′), the Abc2–RecBCD complex was incapable of recognizing or responding to Chi (Murphy, 2000). Plasmids, such as the F-plasmid of E. coli, that enable a cell to donate host chromosomal DNA are called fertility plasmids. Conjugative plasmids of streptococci can mobilize nonconjugative plasmids. As a result, a covalently closed single-stranded copy of the plasmid is introduced into the cytoplasm of the recipient, and its complementary strand is generated by DNA replication in the recipient. More often, bacteria break off after a shorter period of, say, 15–30 minutes, and only part of the chromosome is transferred. Plasmids of Gram-negative bacteria are typically classified into an alphabetical grouping (i.e., IncA through IncZ). Summary of phage recombination functions, P.J. The third category involves conjugative transposons, which are discussed below. Nonetheless, P22 phage containing either an in-frame deletion or an amber mutation in the arf gene showed a 4- to 5-fold decrease in the rate of recombination in P22 phage crosses, a defect that could be complemented by plasmids expressing the arf gene in trans (Poteete et al., 1991). As one bacterium of a mating pair approaches, pilus binding takes place (Fig. This event requires pairs of identical (or nearly identical) DNA sequences, one on the plasmid and the other on the chromosome. It includes components such as= relaxases, relaxosome complex and primase. (adsbygoogle = window.adsbygoogle || []).push({}); Conjugation is brought about by 2 genes in self transmissible plasmid, namely transacting gene (Tra gene) and Origin of transfer (Ori T) site. In order to transfer chromosomal genes, a plasmid must first physically integrate itself into the chromosome of the bacterium. For transfer of chromosome, plasmid must be integrated with chromosome. Conjugative transfer of some plasmids can occur to bacteria or even eukaryotes in which the plasmid cannot replicate, resulting in a transfer range that is much wider than the replication range (Thomas and Smith, 1987). The E. faecalis plasmid is eventually lost from S. aureus, but now the S. aureus lineage carries the vancomycin resistance on its own plasmid, and is thus a VRSA (bottom). A summary of the recombination functions for phages lambda, rac and P22 is listed in Table I. For example, the recipient might have a mutation in the lac operon that prevents growth on lactose as carbon source. 28.14). 2.23b). These plasmids do not show a response to pheromone signals, and the mechanisms of their transfer are unclear. During this process, DNA plasmid is transferred from one bacterium (the donor) of a mating pair into another (the recipient) via a pilus. A model staphylococcal conjugative plasmid, pGO1, contains 52 kb, can transfer among staphylococcal species (possibly by single-stranded transfer), can mobilize small plasmids, and appears to contain an integrated copy of plasmid pUC110 carrying several antibiotic-resistance genes (Clewell, 1993). During bacterial conjugation DNA moves in one direction only, from the plasmid-carrying donor to the recipient (Fig. These catabolic plasmids are usually large (>50 kb) because degradation of organic compounds often requires numerous genes that are part of catabolic cascading pathways. Different Hfr strains will transfer the same genes in different orders and at different times, depending on their location relative to the integration site of the F-plasmid. relaxosome communicates with the coupling protein of Mpf component which signals relaxase when to cut the plasmid at Ori t site, Primase has no role in replication replication of donor plasmid in donor cell. Transferability is the ability of certain plasmids to move from one bacterial cell to another. It is the site where plasmid DNA transfer initiates in donor cell and the site for recyclization in the recipient cell. The tiny bodies covering the F-pili are bacteriophage MS2, a virus that attacks only male bacteria and binds specifically to F-pili. At least two categories of conjugation that are mediated by self-transferable plasmids have been found. J. Lederberg and E. L. Tatum first reported such transfer in 1946 in Escherichia coli. Donor cells are sometimes known as F+ or “male” and recipient cells as F− or “female” and conjugation is sometimes referred to as bacterial mating. [45] by courtesy of Academic Press Inc. B.J. As described earlier, Erf showed projections emanating from its rings, which were identified as the C-terminal domains of the individual subunits (Murphy et al., 1987a). Plasmid Transfer Involving Rolling Circle Replication. The agent responsible for this process was later found to be a site on the chromosome called the F (‘fertility’) factor. After donor and recipient have made contact, the pilus is retracted and the pilus subunits return through the same channel. The length of time it takes for a gene to enter the recipient gives an estimate of its relative distance from the origin of transfer of the Hfr strain. Figure 28.18. Thus, one of the central questions related to plasmid-mediated bacterial evolution is how host ranges and the stability of a plasmid within a host evolve. Certain plasmids, called Tra+ or transfer-positive, are able to move a copy of their DNA into a different cell through a mechanism called bacterial conjugation. From a human perspective it has been transmuted from “female” into a “male”! VirD2 remains covalently bound to the 5′ phosphoryl end of the nicked T-DNA via conserved tyrosine residue Tyr-29. Genes closest to the site of plasmid integration are transferred first (in the order a, b, c, d, e, f, in this example). This male has attached two F-pili to each of the females. Figure 2.23. Magnification: ×11,250. The donor cell manufactures a sex pilus that binds to the recipient and draws the two cells together. One component of the relaxosome, the relaxase, cleaves and remains covalently associated with the 5′ end of the DNA strand destined for transfer (T-strand). By continuing you agree to the use of cookies. A classic example of virulence encoded by plasmids occurs in the Bacillus cereus group of which the species B. cereus, Bacillus thuringiensis, and Bacillus anthracis are all highly genetically related members. When the sex pilus is assembled, its protein subunits travel through the channel in the basal structure (also known as the transfer apparatus). Once the cells are in contact, the basal structure of the pilus makes a connection between the two cells known as the conjugation bridge. Primase is trnasfered to the recipient cell and synthesizes a primer to complete the replication of another strand of plasmid DNA in recipient cell. Most plasmids are circular dsDNA but linear plasmids also exist (Stewart et al., 2005; Hinnebusch and Tilly, 1993); they typically consist of (1) a ‘backbone,’ which contains the genes necessary for self-replication, maintenance, control, and conjugative transfer and (2) various ‘accessory’ genes that provide other functions to the host bacterium. Notice that the transfer of the single-stranded DNA does not end with the F-plasmid DNA and continues into the chromosomal DNA. Conjugation in bacteria is a process in which plasmids are transferred by themselves alone or along with other DNA element from one cell to another cell through conjugation tube. It has 3 components- pilus, channel and coupling protein, Pilus holds donor and recipient cell together.