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7.11F: Gene Transfer in Archaea

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    9302
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    Archaea are distinct from bacteria and eukaryotes, but genetic material can be transferred between them and between Archaea themselves.

    LEARNING OBJECTIVES

    Describe the mechanisms of gene transfer in Archaea

    KEY TAKEAWAYS

    Key Points

    • Archaea while being very different from eukaryotes and bacteria, there are many commonalities at the the genetic level between them.
    • Horizontal gene transfer can explain the similarities between the genes found in the three domains of life and indeed there is evidence that horizontal gene transfer occurs with Archaea species.
    • Archaea can be infected by double-stranded DNA viruses, which can account for gene transfers, as well like bacteria, Archaea may conjugate.

    Key Terms

    • archaea: a taxonomic domain of single-celled organisms lacking nuclei that are fundamentally from bacteria.
    • translation: Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text.
    • transcription: Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes.

    Archaea are genetically distinct from bacteria and eukaryotes, but are poorly understood: many of the genes that Archaea encode are of unknown function. Transcription and translation in archaea resemble the same processes more closely in eukaryotes than in bacteria, with the archaean RNA polymerase and ribosomes being very close to their equivalents in eukaryotes.

    Although Archaea only have one type of RNA polymerase, its structure and function in transcription is similar to that of the eukaryotic RNA polymerase II, with similar protein assemblies (the general transcription factors) directing the binding of the RNA polymerase to a gene’s promoter. However, other archaean transcription factors are closer to those found in bacteria. Post-transcriptional modification is simpler than in eukaryotes, since most archaean genes lack introns, although there are many introns in their transfer RNA and ribosomal RNA genes, and introns may occur in a few protein-encoding genes. This is all to say there are many similarities in the genes shared between Archaea and the other domains of life, suggesting there was a transfer of genetic material between the domains of life. This phenomenon is described as horizontal gene transfer.

    Horizontal gene transfer (HGT) refers to the transfer of genes between organisms in a manner other than traditional reproduction. Also termed lateral gene transfer, it contrasts with vertical transfer, the transmission of genes from the parental generation to offspring via sexual or asexual reproduction. HGT has been shown to be an important factor in the evolution of many organisms, including bacteria, plants and humans.

    Archaea show high levels of horizontal gene transfer between lineages. Some researchers suggest that individuals can be grouped into species-like populations given highly similar genomes and infrequent gene transfer to/from cells with less-related genomes, as in the Archaea genus Ferroplasma. On the other hand, studies in Halorubrum found significant genetic transfer to/from less-related populations. These gene transfers are identified by sequencing the DNA of various Archaea species; through the similarities and differences of the DNA of the different types of Archaea it is determined if the gene was perfectly transferred or from a common ancestor. The elucidation of this can be controversial.

    How genetic material can move from one Archaea to another is poorly understood. In bacteria the natural ways in which this occurs is through either bacterial conjugation or viral transfer, also known as transduction. Conjugation is where two (sometimes distantly related) bacteria transfer genetic material by direct contact. Transduction occurs when a virus “picks up” some DNA from its host and when infecting a new host, moves that genetic material to the new host. It is thought that conjugation can occur in Archaea, though unlike bacteria the mechanism is not well understood. As well Archaea can be infected by viruses. In fact Archaea can be infected by double-stranded DNA viruses that are unrelated to any other form of virus and have a variety of unusual shapes, including bottles, hooked rods, or teardrops. Taken together it is clear that gene transfer happens in Archaea, and probably is similar to horizontal gene transfer seen in the other domains of life.

    image

    Archaeal viral infection: Cell of Sulfolobus infected by virus STSV1 observed under microscopy. Two spindle-shaped viruses were being released from the host cell. The strain of Sulfolobus and STSV1 (Sulfolobus tengchongensis Spindle-shaped Virus 1) were isolated by Xiaoyu Xiang and his colleagues in an acidic hot spring in Yunnan Province, China. At present, STSV1 is the largest archaeal virus to have been isolated and studied. Its genome sequence has been sequenced.

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