Transfer of Plasmids between Bacteria

Transferability is the ability of certain plasmids to move themselves from one bacterial cell to another. Many medium sized plasmids, such as the F-type and P-type plas-

attl Lambda attachment site—site where lambda inserts its DNA into the bacterial chromosome defective phage Mutant phage that lacks genes for making virus particles helper phage Phage that provides the necessary genes so allowing a defective phage to make virus particles transferability Ability of certain plasmids to move themselves from one bacterial cell to another mids, can do this and are referred to as Tra+ (transfer positive). For transfer to occur, the two bacterial cells must come into physical contact and move the DNA by a process known as bacterial conjugation. DNA is transferred in one direction only, from the plasmid-carrying donor to the recipient (Fig. 18.11). The donor cell manufactures a sex pilus that binds to a suitable recipient and draws the two cells together. Next, a conjugation bridge forms between the two cells and provides a channel for DNA to move from donor to recipient. In real life, mating bacteria actually tend to cluster together in groups of five to ten (Fig. 18.12).

The genes for formation of the sex pilus and conjugation bridge and for overseeing the DNA transfer process are known as tra genes and are all found on the plasmid itself. Since plasmid transfer requires over 30 genes, only medium or large plasmids possess this ability.Very small plasmids, such as the ColE-plasmids, do not have enough DNA to accommodate the genes needed. Plasmids that enable a cell to donate DNA are called fertility plasmids and the most famous of these is the F-plasmid of E. coli, which is approximately 100 kbp long. 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. Note however, that the "sex" of a bacterial cell is determined by the presence or absence of a plasmid and that DNA transfer is unidirectional, from donor to recipient. When a recipient cell has received the F-plasmid it becomes F+. From a human perspective it has been transmuted from "female" into a "male"! Thus bacterial mating is not at all equivalent to sexual reproduction among higher organisms.

Plasmid transfer actually involves replication by the rolling circle mechanism (Fig. 18.13). First one of the two strands of the double stranded DNA of the F-plasmid opens up at the origin of transfer. This linearized single strand of DNA moves through the conjugation bridge from the donor into the recipient cell. An unbroken single stranded circle of F-plasmid DNA remains inside the donor cell. This is used as a template for the synthesis of a new second strand to replace the one that just left. As the linear single strand of F-plasmid DNA enters the female cell, a new complementary strand of DNA is made using the incoming strand as template. Thus only one strand of the F-plasmid DNA is transferred from the donor to the recipient.

Although ColE and other small plasmids are not self-transferable, they are often mobilizable (Mob+). A transferable plasmid, such as the F-plasmid, can mobilize the ColE plasmid if they both inhabit the same cell. The F-plasmid oversees conjugation and forms the conjugation bridge and the ColE plasmid is transferred through this. The mob (mobilization) genes of the ColE plasmid are responsible for making a single-stranded nick at the origin of transfer of ColE and for unwinding the strand to be transferred.

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