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Getting Into Yeast’s Genes

September 26, 2013

Yeast has been responsible for a lot of hook ups in its day (think beer goggles and margaritas on the beach).  Now it is payback time.  In a new study, Giraldo-Perez and Goddard have figured out how to make yeast more promiscuous.

If he were a yeast, he’d harbor the VDE homing endonuclease.

No, they don’t get the yeast drunk.  Instead, they found that strains containing VDE, a homing endonuclease gene (HEG), entered meiosis more often than genetically identical strains that lacked VDE.  The yeast that contained this “selfish” gene (well, actually intein) were ready to go haploid more often than those that didn’t.

VDE and its ilk are said to be selfish because they end up getting passed down to more offspring than a certain Austrian monk might have predicted.  When a diploid is heterozygous for an HEG, the homing endonuclease cuts the sister chromosome at the equivalent spot. Then, when the diploid undergoes meiosis, the sister is repaired through recombination causing both chromosomes to contain the VDE gene.  Now instead of two spores containing VDE, all four will.

Giraldo-Perez and Goddard monitored the percentage of sporulating cells over a 30 day period and found that after five days, a higher percentage of diploids homozygous for VDE sporulated compared to diploids heterozygous for or lacking VDE.  The authors contend that under the right conditions, this increased sporulation would allow VDE to spread through a population 20 times faster than it might otherwise.  And the authors found that VDE needs something like this or it might disappear.

Like alcohol, VDE isn’t all lowered inhibitions and good times.  For example, yeast homozygous for VDE grow significantly more slowly than do yeast lacking VDE in YPD, grape juice, vineyard soil, vine bark (heterozygotes fall in between).  This obviously puts yeast carrying VDE at a disadvantage, meaning that if it didn’t have another trick up its sleeve, it would dwindle away to nothing.  That trick is speeding up sporulation. 

The authors weren’t able to determine why this little bit of DNA can have such a profound effect on the growth rate of yeast.  It is almost certainly too little DNA to affect the time it takes the yeast to copy its DNA.  And the endonuclease itself is probably not randomly nicking the chromosomal DNA in the mitotic state, since it is kept out of the nucleus by host encoded karyopherins.

So VDE is a truly a parasitic selfish gene.  It is parasitic because it sucks a little of the life out of a yeast cell.  And it is selfish because way more daughters end up with it than might be predicted.  Sounds like a nice description for many drunk people…