U. of Virginia study: Archaeal horizontal gene transfer similar to that of bacteria
Horizontal gene transfer is the movement of genetic material by means other than the transmission of DNA from parent to offspring. It can occur in both single and multicelled organisms, but has been most closely studied in bacteria, where there are three known ways that it can occur, transformation, conjugation and transduction.
Horizontal gene transfer is the movement of genetic material by means other than the transmission of DNA from parent to offspring. It can occur in both single and multicelled organisms, but has been most closely studied in bacteria, where there are three known ways that it can occur, transformation, conjugation and transduction.
When horizontal gene transfer occurs by conjugation, two cells are joined, one a donor and the other a recipient. When this mechanism occurs in infectious bacteria, it allows them to foster the spread of antibiotic resistance.
The machinery for conjugation in bacteria is well characterized in the scientific literature, but much less is known about archaeal conjugation. Archaea are single-celled organisms that at one time were classified as bacteria but currently are considered a separate domain of cellular life.
Recently, an international group of researchers has used cryo-electron microscopy to examine how conjugation machinery operates in archaea. The DNA exchange apparatus for this process is a cell appendage known as a mating pilus.
The researchers were able to observe the atomic structures of conjugative pili from two types of archaea that live in extremely hot environments and from one bacterium.
The authors show that the archaeal mating pili are homologous to the bacterial mating pili, and they investigate many features of conjugation. (Homologous refers to processes that are thought to be similar in origin.) One difference they note is that the genes for the archaeal conjugation are encoded on the chromosomes, and are not mobile genetic elements.
The research appears in Nature Communications, Feb. 7. The lead author, Leticia Beltran, is at the Department of Biochemistry and Molecular Genetics at the University of Virginia in Charlottesville.
An important mechanism for life
The authors frame their work by noting that the "importance of horizontal gene transfer (HGT) in microbial persistence and evolution cannot be overstated." Without this genetic exchange, they write, microbial populations would die off, because they could not reverse the buildup of "deleterious mutations."
To avoid extinction both bacteria and archaea developed specific apparatus for horizontal gene transfer: transformation, transduction and conjugation.
The researchers studied the pili of two hyperthermophilic archaea (Aeropyrum pernix and Pyrobaculum calidifontis). These were compared with pili of the bacerium Agrobacterium tumefaciens.
The particular DNA transfer system developed in the hyperthermophilic archaea is called crenarchaeal exchange of DNA or Ced, which is induced by ultraviolet radiation. The Ced mechanism mediates the import of DNA which then is involved in genome repair.
Those cells that do not exchange DNA, they report, "show significantly lower survival rates upon DNA damage."
Although this Ced mechanism was thought to be unrelated to the conjugation system of bacteria, which used export instead of import of DNA, the researchers found otherwise.
"Here using cryo-electron microscopy (cryo-EM), we show that a protein from the hyperthermophilic archaeon Aeropyrum pernix, a homolog of CedA1, forms a pilus which is structurally homologous to bacterial conjugative pili," they wrote. "We also discover that structurally similar pili, although with no sequence similarity, are produced by members of the Thermoproteales, which were previously not considered to encode the Ced-like system.
"We present high-resolution structures of two putative conjugative pili from hyperthermophilic archaea, A. pernix and Pyrobaculum calidifontis, and a bacterial conjugation pilus from a model system encoded by the pTiC58 plasmid of Agrobacterium tumefaciens," they added.
The authors conclude that the conjugation systems of these archaea and the bacterium share a common ancestor. But the hyperthermophilic archaea conjugation system has been "domesticated," that is, encoding the genes for the mechanism on the chromosome, they note, instead of being on a plasmid. The researchers propose that this domestication evolved as a way to help the archaea survive in their extremely hot environment.
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Leticia C. Beltran et al. "Archaeal DNA-import apparatus is homologous to bacterial conjugation machinery." Nature Communications, Feb. 7.
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