BACTERIAL CONJUGATION: PROCESS, STRUCTURE AND FACTORS - BIOLOGY - 2024

The bacterial conjugation is to transfer in one direction of genetic material from a donor bacterium to another recipient, by physical contact between the two cells. This type of process can occur both in bacteria that react, as in those that do not react to Gram stain, and also in streptomycetes.

Conjugation can occur between bacteria of the same species, or of different species. It can even occur between prokaryotes and members of other kingdoms (plants, fungi, animals).

Bacterial conjugation. The image shows, from top to bottom, two bacteria before, during and after conjugation. Taken and edited from User Magnus Manske on en.wikipedia.

For the conjugation process to occur, one of the bacteria involved, the donor, must possess the genetic material that can be mobilized, which is generally represented by plasmids or transposons.

The other cell, the recipient, must lack these elements. Most plasmids can detect potential recipient cells that lack similar plasmids.

Conjugation and sexual reproduction

Bacteria do not have an organization of genetic material similar to that of eukaryotes. These organisms do not present sexual reproduction because they do not present reductional division (meiosis) to form gametes at any time in their life.

To achieve the recombination of their genetic material (essence of sexuality), bacteria have three mechanisms: transformation, conjugation and transduction.

Bacterial conjugation is not, then, a process of sexual reproduction. In the latter case, it can be considered a bacterial version of this type of reproduction, since it involves some genetic exchange.

Structures and factors involved in the process

Sexual pili

Also called pili F, they are filamentous structures, much shorter and thinner than a flagellum, made up of protein subunits intertwined around a hollow center. Its function is to keep two cells in contact during conjugation.

It is also possible that the conjugative element is transferred to the recipient cell via the central foramen of the sex pili.

Conjugative elements

It is the genetic material that will be transferred during the bacterial conjugation process. It can be of a different nature, among them are:

Extrachromosomal DNA particles

These particles are episomes, that is, plasmids that can be integrated into the bacterial chromosome through a process called homologous recombination. They are characterized by having a length of approximately 100 kb, as well as by having their own origin of replication and transfer.

Cells that possess factor F are called male cells or F + cells, while female cells (F-) lack this factor. After conjugation, the F- bacteria become F + and can act as such.

Chromosome Strands

When homologous recombination occurs, factor F binds to the bacterial chromosome; in such cases it is called factor F 'and the cells that have the recombinant DNA are called Hfr, for high frequency recombination.

During conjugation between an Hfr bacterium and an F- bacterium, the first transfers to the second a strand of its recombinant DNA with factor F. In this case, the recipient cell itself becomes an Hfr cell.

There can only be one factor F in a bacterium, either in extrachromosomal form (F) or recombined to the bacterial chromosome (F ').

Plasmids

Some authors consider plasmids and F factors together, and other authors treat them separately. Both are extrachromosomal genetic particles, but unlike factor F, plasmids do not integrate into chromosomes. They are the genetic elements that are mostly transmitted during the conjugation process.

Plasmids are composed of two parts; a resistance transfer factor, which is responsible for the transfer of the plasmid, and another part made up of multiple genes that have the information that encodes resistance to different substances.

Some of these genes can migrate from one plasmid to another in the same cell, or from a plasmid to the bacterial chromosome. These structures are called transposons.

Some authors argue that plasmids beneficial to bacteria are actually endosymbionts, while others may, on the contrary, be bacterial endoparasites.

Process

The donor cells produce the sex pili. The F particles or plasmids present only in these bacteria, contain the genetic information that encodes the production of the proteins that form the pili. Because of this, only F + cells will present these structures.

Sex pili allow donor cells to attach to recipient cells first and then to stick together.

In order to initiate the transfer, the two strands of the DNA strand must be separated. First, a cut occurs in the region known as the origin of transfer (oriT) of one of the strands. A relaxase enzyme makes this cut so that later a helicase enzyme begins the process of separating both chains.

The enzyme can act alone or also by forming a complex with several different proteins. This complex is known by the name of relaxosome.

Immediately starting the separation of the chains, the transfer of one of the strands will begin, which will only end when the complete strand has passed to the recipient cell, or when the two bacteria separate.

To complete the transfer process, both cells, recipient and donor, synthesize the complementary strand, and the chain is made to circulate again. As a final product, both bacteria are now F + and can act as donors with F- bacteria.

Plasmids are the genetic elements most frequently transmitted in this way. The conjugation capacity depends on the presence in the bacterium of conjugative plasmids that contain the genetic information required for such a process.

Applications

Conjugation has been used in genetic engineering as a tool to transfer genetic material to different destinations. It has served to transfer genetic material from bacteria to different receptor eukaryotic and prokaryotic cells, and even to isolated mammalian mitochondria.

One of the genera of bacteria that has been used most successfully to achieve this type of transfer is Agrobacterium, which has been used alone, or in conjunction with the tobacco mosaic virus.

Among the species genetically transformed by Agrobacterium are yeasts, fungi, other bacteria, algae and animal cells.

Transformation by Agrobacterium tumefaciens to a plant cell. Taken and edited from: J LEVIN W.

References

1. EW Nester, CE Roberts, NN Pearsall & BJ McCarthy (1978). Microbiology. 2nd edition. Holt, Rinehart and Winston.
2. C. Lyre. Agrobacterium. In lifeder. Recovered from lifeder.com.
3. Bacterial conjugation. On Wikipedia. Recovered from en.wikipedia.org.
4. R. Carpa (2010). Genetic recombination in bacteria: horizon of the beginnings of sexuality in living organisms. Elba Bioflux.
5. Prokaryotic conjugation. On Wikipedia. Recovered from es.wikipedia.org.
6. LS Frost & G. Koraimann (2010). Regulation of bacterial conjugation: balancing opportunity with adversity. Future Microbiology.
7. E.Hogg (2005). Essential Microbiology. John Wiley & Sons Ltd.