Hfr strain

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A bacterial strain that is characterized by a high rate of recombination is referred to as an Hfr strain (abbreviation for high frequency of recombination ) .

properties

Recombination usually takes place after conjugation , in which cell contact is mediated by a sex pilus (also called F-pilus ). The DNA is then transferred from one cytosol to the next via a cytoplasmic bridge . In Mycobacterium smegmatis , a different mechanism of DNA transfer has been described.

An example of Hfr strains are bacteria with F plasmid . The F plasmid is integrated into the chromosome of such a cell , which leads to an increased activity of the F factor. F + denotes a cell that has an F plasmid and thus has the ability to transfer DNA. A cell without an F plasmid is referred to as F - . During the conjugation there is a transfer of DNA from F + to F - cells. However, in Hfr clones it is not the F plasmid that is transferred, but parts of the bacterial chromosome, which is why the recipient cell does not become a donor cell. This process is very prone to failure and is interrupted even with slight vibrations. In most cases, only about 1000 bases of a single DNA strand are transferred. Through homologous recombination , the recipient cell can integrate the transferred genes into its own chromosome.

The first gene maps for Escherichia coli were produced using interrupted conjugation experiments. During the conjugation, different Hfr strains as DNA donors were separated from the DNA recipients by means of mechanical shear forces in a stand mixer ( Waring blender ). The duration of the conjugations was measured and it was investigated which genes were transferred at what time. That is why these gene maps are scaled in minutes.

By cutting out ( excising ) the F factor from the chromosome of an Hfr cell, an F 'plasmid is created, which, depending on the interfaces, can also contain genes of the bacteria. The Hfr cell becomes an F + cell again. These bacterial genes, which were transferred to the F 'plasmid, are, however, lost to the bacterial chromosome. If the tra genes are completely retained during the excision, the newly formed F 'plasmid can be transferred back to F - cells by conjugation . As a result, with regard to the genes that were taken along when the F plasmid was transferred, merodiploid cells, also known as merozygotes , can arise.

literature

  • K. Brooks Low: Hfr Strains of Escherichia coli K-12 . In: Frederick C. Neidhardt (ed.) Escherichia coli and Salmonella typhimurium : Cellular and Molecular Biology, ASM Press, Washington, DC, 1986, pp. 1134-1137, ISBN 0-914826-89-1

Individual evidence

  1. ↑ LS Baron, WF Carey, WM Spilman: Characteristics of a high frequency of recombination (HFR) strain of Salmonella typhosa compatible with Salmonella Shigella and Escherichia species. In: Proceedings of the National Academy of Sciences . Volume 45, Number 12, December 1959, pp. 1752-1757, ISSN  0027-8424 . PMID 16590568 . PMC 222794 (free full text).
  2. ↑ J. Wang, PK Karnati, CM Takacs, JC Kowalski, KM Derbyshire: Chromosomal DNA transfer in Mycobacterium smegmatis is mechanistically different from classical Hfr chromosomal DNA transfer. In: Molecular microbiology. Volume 58, Number 1, October 2005, pp. 280-288, ISSN  0950-382X . doi : 10.1111 / j.1365-2958.2005.04824.x . PMID 16164565 .