Thursday, January 13, 2011

Gene Transfer (part III)

(Click here for part I--transduction, or here for part II--gene transfer agents)

There’s a lot we don’t know about GTAs—and there’s lots of evidence that they may be hugely important in the evolution of many Bacterial and Archaeal lineages. A report from October of 2010 finally provides some solid clues to just how important GTAs are for “wild” cells. Up to this time, GTAs had been either studied in the lab, or merely predicted from analysis of genome data. A group led by John H. Paul at the University of Florida decided to see if GTAs actually worked in “wild” cells. This required a little genetic manipulation—so the study is not absolutely pure—but their results show the importance of GTAs in natural systems.

The first thing that Paul’s group did was to find “wild” cells that could produce GTAs. They added a few easily detected genes to these cells—specifically, genes for antibiotic resistance. If GTAs actually moved genes from one cell to another, then GTAs from these cells could make “recipient” cells resistant to antibiotics, and thus very easy to find. However, they wanted to see if GTAs worked in the wild. So, they mixed together GTAs from their donor cells and samples of seawater in the natural environment. The photo shows their experimental apparatus, which contained the cells so that no GMOs would be released into the environment.

After a day, they checked to see if any of the cells in their seawater sample became resistant to antibiotics. Sure enough, many did—as many as one in 100 cells.

This looks like proof that GTAs could transfer genes “in the wild.” However, the researchers still needed to eliminate an alternative hypothesis—that the cells in their seawater spontaneously became resistant to antibiotics. This did actually happen, but at a much, much lower rate—when they tried the same experiment without any GTAs, they found that 1 in 10,000 cells spontaneously became antibiotic resistant.

This finding forces us to re-evaluate how often genes get shared among different organisms in the real world. Paul’s group found that GTAs could efficiently transfer genes between fairly dissimilar types of bacteria. This is something viruses are not so good at (and it's also the reason that you don’t have to worry about catching a viral infection from frogs). Also, they found that the efficiency of gene transfer by GTAs is about a million times that of transduction. So, in a very real sense, the ocean is swimming with genes, and species evolve influenced not just by the physical parameters of temperature and light, but also by the genes that are there for the taking. Genomes are far more flexible than we previously thought, constantly acquiring new genes and trying them out, and evolving with astonishing speed. Evolution is not an orderly, linear march; it is a spastic tango, darting and weaving and messily exchanging genes all over the place.

Lang, Andrew S, and J. Thomas Beatty (2006). Importance of widespread gene transfer agent genes in a-proteobacteria. TRENDS in Microbiology 15: 2, 54-62.

Marrs, Barry (1974). Genetic Recombination in Rhodopseudomonas capsulata. Proceedings National Academy of Science USA 71:3, 971-973.

McDaniel, Lauren, Elizabeth Young, Jennifer Delaney, Fabian Ruhnau,

Kim B. Ritchie, and John H. Paul (2010). High Frequency of Horizontal Gene Transfer in the Oceans. Science 330, 50.

Solioz, Marc, Huei-Che Yen, and Barry Marrs (1975). Release and Uptake of Gene Transfer Agent by Rhodopseudomonas capsulata. Journal of Bacteriology 123:2, 651-657.

Yen, H.C., N. T. Hu, and Barry L. Marrs (1979) Characterization of the Gene Transfer Agent Made by an Overproducer Mutant of Rhodopseudomonas capsulata. Journal of Molecular Biology 131, 157-168.

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