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Horizontal transfer of genes occurred across all domains of life to fend off bacteria

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Posted December 3, 2014

Horizontal gene transfer has long been recognized between different kinds of organisms, especially the microbes. However, no single gene has been previously identified as universally shared across the domains of life – bacteria, archaea and eukaryotes.

The tree of life. Credit: Samsara via Wikimedia Commons (CC BY-SA 3.0).

The tree of life. Credit: Samsara via Wikimedia Commons (CC BY-SA 3.0).

 

Now a new research suggests for the first time that a family of antimicrobial genes could have spread horizontally across all branches of the tree of life and done so successfully due to a universal need to fight off bacteria.

A study published on eLife last week demonstrates that lysozyme family enzymes, which possess broad-spectrum antibacterial properties, have experienced multiple species-to-species transfers in the course of evolution. These events were likely selected-for due to universal presence of competing and / or pathogenic bacteria in different ecological contexts.

Theoretically, horizontal gene transfer can occur between most organisms. Gene exchange happens often between different bacteria (hence the rapid spread of antibiotic resistance genes), and has also been increasingly documented between two domains (e.g., bacteria and eukaryotes) or between viruses and their hosts. Transfers across all three domains of life, however, are least likely.

For a gene to be shared successfully, environmental and species barriers must be crossed, not to mention a high selective advantage that is required for a gene to be maintained in the recipient organism. The authors of the study argue that lysozymes, which catalyze breakdown of bacterial cell walls, could have provided exactly that.

Bacteria are present everywhere and, in one way or another, a large proportion of them tends to cause trouble to their neighbors. Archaea and certain eukaryotes (e.g., fungi) may compete with bacteria for resources, larger organisms (e.g., plants and animals) are subject to bacterial infections and so on. Therefore, an ability to produce antibiotics is certainly a handy tool in any ecological niche.

Likely events of horizontal gene transfer across the three domains of life (colored lines). Image courtesy of the researchers.

Likely events of horizontal gene transfer across the three domains of life (colored lines). Image courtesy of the researchers.

Curiously though, the origin of most antibiotic genes can be tracked to the bacteria themselves. For example, lysozymes, also known as muramidases, are important enzymes used in cell division and to remodel bacterial cell walls as required by the host. However, the same enzyme in a competing organism can be turned against bacteria by digesting their walls prematurely.

Most importantly, the research describes the first antibacterial gene in archaea – a diverse group of organisms, which has so far been ignored in antibiotic discovery research. Archaea are known to coexist with bacteria in a variety of environments, and have been observed to inhibit their competitors. As such, this group of microorganisms could represent a potential, yet unexplored, source of novel antibacterial therapeutics, which is especially important in a current antibiotics crisis. Moreover, due to an ability of some archaea to thrive in extreme environments, such as hot streams, archaea-derived antibiotics could have additional properties, such as thermal stability, making the therapeutics even more attractive.

Based on their findings, researchers predict future studies to reveal even more universally shared antimicrobial genes, since the pressure to survive in the “bacterial world” provides solid ground for successful horizontal gene transfer across all domains of life.

Written by Eglė Marija Ramanauskaitė

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