Conjugation (biology)
In microbiology, conjugation (in Latin coniugare to bind together in pairs) describes the transfer of parts of the genome from a donor cell (donor) to a recipient cell (recipient) through direct cell contact. In addition, the term is used in mycology , the study of fungi, for the sexual fusion of specialized fungal hyphae .
Conjugation in prokaryotes
The conjugation in prokaryotes is an example of parasexuality and is also possible with them far beyond species boundaries. For example, genes for antibiotic resistance , toxins and colonization factors can be transferred from a species harmless to humans to a pathogenic species. Genes for the formation of root nodules and bacteriocins are also transferred by conjugation. There are also cases where DNA is transferred from prokaryotes to eukaryotes, e.g. B. transmits the bacterium Agrobacterium tumefaciens so-called. Ti plasmids in wounded areas of certain plants. In experiments, conjugations of Escherichia coli on baker's yeast, Saccharomyces cerevisiae and on egg cells from hamsters were also detected.
The process of conjugation was discovered in 1946 by Joshua Lederberg and Edward Tatum in Escherichia coli .
Gram negative bacteria
Conjugation in gram-negative bacteria is controlled by plasmids , mostly self-contained DNA molecules with non-essential genes. Conjugative plasmids carry the specific information of the conjugation mechanisms, ie the genes coding for it in the tra region (tra for transfer), including a relaxase for the single-stranded cut of the DNA to be transferred. Not all plasmids have a tra region, so there are also non-conjugative plasmids. A donor cell is a cell with a conjugative plasmid; it can use an F-pilus , a thread-like cell appendage, which is also encoded by genes on the plasmid, to make contact with a cell that does not have this plasmid, the recipient. After contact is made, the pilus is broken down until the two cells touch. A conjugation bridge is formed at the point of contact, the structure of which has not yet been clarified. Through this one of the two DNA strands of the plasmid is transferred into the recipient. In the case of circular plasmids, this usually takes place via the rolling circle mechanism , in which a DNA strand is broken at a specific point, "unrolled" and transferred to the recipient. Already during the unwinding, the released strand is replaced by synthesis in the donor. After the transfer, the single strand in the recipient is supplemented to form a double strand and the plasmid form is adopted. At the end of the conjugation, both cells have the conjugative plasmid and are therefore both donors.
When transferring the plasmid strand, further plasmids, including non-conjugative ones, can also be transferred. However, these require so-called mob genes in order to also use the pilus. This phenomenon is called "mobilizability".
Gram positive bacteria
Gram-positive bacteria can also mediate conjugation via conjugative transposons instead of via plasmids . In contrast to the gram-negative bacteria, they do not form sex pili. The potential recipients secrete pheromones , which stimulate donors to form what is known as an aggregation protein through which the two partners can attach themselves to each other. Then DNA can be transferred. The processes of conjugation are even less researched in gram-positive bacteria than in gram-negative bacteria.
Archaea
Conjugative plasmids have also been discovered in archaea of the genus Sulfolobus . The exact process of the conjugation is hardly known here. Since the genetic makeup of the plasmids seems to differ considerably from that of bacterial plasmids, it is assumed that the mechanisms of archaeal conjugation also differ significantly from those of bacterial conjugation.
Episomes
If a plasmid has certain sequences in the DNA, insertion sequences (IS), which are homologous to sequences in the bacterial chromosome, the plasmid can be incorporated into the chromosome. Such plasmids are called episomes . Cells that have an episome located in the chromosome are called hfr (high frequency of recombinations). Since the plasmid genes are still present, conjugations can also take place, but now chromosomal genes are also transferred in addition to some of the plasmid genes. These can be recombined with their own DNA in the recipient . Since not all plasmid genes are usually transferred, the recipient in this case of conjugation does not become the donor.
Transfer of chromosomal genes into the plasmid
Plasmids integrated into the chromosome can also be cut out again there. Since there are often several homologous IS in the chromosome, chromosomal parts can also be cut out with the plasmid and integrated into the plasmid. This modified plasmid also transfers the chromosomal elements during conjugation, resulting in a partial diploidy during conjugation within the same strain . Since the recipients receive the entire plasmid here, they become donors after conjugation.
F plasmid
The conjugation is usually explained using the example of the F plasmid (F for fertility), which occurs in the gram-negative model organism Escherichia coli . The Sexpili as F-pili are referred to, the donor is F + and the recipient F - called. The location on the plasmid where transfer begins is the oriT sequence . An F plasmid cut out with chromosomal portions is referred to as an F 'plasmid.
Gene mapping
The time dependence of the transmission of the DNA of an hfr strain can be used to determine the order in which the genes are located on the chromosome. For this purpose, hfr strains and recipients with known properties are used. The transfer of the DNA to the recipient is stopped at different time intervals by mechanical forces. Through minimal media , i.e. H. Culture media on which only organisms with certain properties can multiply, can then be determined which genes have been transferred. The time intervals enable the classification of where the gene is located on the chromosome, which is why older gene maps are often divided into minutes. The Escherichia coli gene map was determined by this method long before sequencing .
Conjugation in ciliates
When conjugation in ciliates is a special form called sexual processes in the unicellular ciliates ( Ciliophora ). Two individuals lie next to each other and form a so-called plasma bridge through which genetic material is exchanged. Conjugation and exchange only take place between individuals belonging to different mating types. This prevents members of the same mating type from exchanging genetic material. The mating types are defined by glycoproteins on the surface.
In ciliates, the genetic material consists of a micronucleus and a macronucleus. During conjugation, the macronucleus gradually dissolves and the micronuclei of both partners result in four haploid nuclei each through the two processes of division of meiosis. With the exception of one of these haploid nuclei, all nuclei thus created also dissolve again. The two remaining nuclei now divide in a further mitosis into two haploid nuclei, a stationary nucleus and a wandering nucleus. The stationary core, also known as the female core, remains in the respective individual; the migrating nucleus, also known as the male nucleus, penetrates the conjugation partner via the plasma bridge and merges there with its stationary nucleus. So every individual now has a diploid nucleus.
After the two sexual partners have separated, the diploid nucleus is doubled through a further mitosis, the macronucleus is built up from one of the two daughter nuclei through polyploidization, the other daughter nucleus remains unchanged as a micronucleus.
See also
swell
- Brock: Mikrobiologie , 11th edition 2009, chap. 10.2.3 - 10.2.6
- John Wöstemeyer: Microbiology , 2009, p 33/34
- Georg Fuchs (Ed.): General microbiology , 8th edition 2007, chap. 15.6.2
- Paul Singleton: Introduction to Bacteriology , 1995, chap. 8.4.2