GTA mediated horizontal gene transfer: Towards understanding the influencing factors

The distribution of successful genetic mutations is paramount to the survivability of every living thing, and whilst bacteria perform no sexual reproduction, it has long been known that genetic transfer through conjugation, transformation and transduction has enabled the adaptation of bacteria to their surround environment.

Recently a fourth mechanism of genetic transfer has been discovered, which has explained the rapidity of intraspecies horizontal gene transfer. This method is mediated by small virus-like particles called gene transfer agents (GTAs), which are seemingly random segments of the hosts DNA, packaged into protein coats and are released into the surrounding environment to be taken up and incorporated by other bacteria. At the present level of research it is unknown what are the influencing factors on the release of GTAs, what species of bacteria can perform this method of gene transfer (although it has been demonstrated in four species of alpha-proteobacteria) and exactly how the GTA is released (but is thought to be in response to a lytic event of some of the population). However it is understood that GTAs are spontaneously released in the stationary phase and that they vary greatly in size, morphology and the amount of DNA they package.

This paper aims to define some of the influencing factors responsible for the release of GTAs. This was studied through the creation of Tn5 mutants of samples collected at different geographical locations across various times in the year. Tn5 is an antibiotic resistance gene encoding the resistance against kanamycin and streptomycin. This gene was used for ease of identification of positive mutants.

An exact match of the transposed Tn5 gene was found in 11% of the screened colonies, of which no significant correlation was observed between bacteria found in different geographical locations. However significant observations found show a higher release of GTAs in cooler water conditions, and that GTA release is predominantly impacted by salinity levels.  No explanation was offered by the author as to why this abiotic factor has such an impact.

The author concludes that these results indicate that GTA mediated horizontal gene transfer is favoured during times of elevated bacterial abundance and salinity. In addition to this the authors speculated that up to 0.05% of marine viruses could be GTAs.

I believe this study is an important insight into the lesser studied fourth method of bacterial gene transfer and further research in this field is vital to understand the exact mechanisms and species which are able to perform this. Describing the molecular mechanisms may contribute to our understanding of bacterial pathogenicity and epidemiology, and thus may save human lives in the future. 

REFERENCE: McDaniel LD, Young EC, Ritchie KB, Paul JH (2012) Environmental Factors Influencing Gene Transfer Agent (GTA) Mediated Transduction in the Subtropical Ocean. PLoS ONE 7(8): e43506. doi:10.1371/journal.pone.0043506

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