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Gender Bending in Yeast

December 3, 2013

Even after all these years of studying the mating response in yeast, there is still more to be learned! Image courtesy of Lori B. Huberman and GENETICS

Our friend Saccharomyces cerevisiae has it pretty easy when it comes to sex.  There is no club scene or online dating.  Pretty much if an a and an α are close enough together, odds are that they will shmoo towards each other and fuse to create a diploid cell.  No fuss, no muss.

Of course there aren’t any visual cues that indicate whether a yeast is a or α.  Instead yeast relies on detecting gender-specific pheromones each cell puts out.  The a yeast makes a pheromone and an α pheromone receptor, and the α yeast makes α pheromone and an a pheromone receptor.  The way yeast finds a hottie is by looking for the yeast of the opposite sex that puts out the most pheromone.  

This simple system is similar to ours in that gender is determined by gender specific gene expression.  In humans this happens through the amounts of certain hormones that are made.  For example, males make a lot of testosterone which turns on the androgen receptor (AR) which then turns a bunch of genes up or down.  Both men and women have AR; men just make more testosterone, which causes it to be more active. 

Yeast are simpler in that their mating loci encode transcription factors and cofactors that directly regulate a-specific and α-specific genes. Still, in both yeast and human, gender is determined by which genes are on and which are off.

Given how simple the yeast system is and how extensively it has been studied, you might think there is nothing else to learn about yeast mating.  You’d be wrong.  In a new study out in GENETICS, Huberman and Murray found that a gene with a previously unknown function, YLR040C, is involved in mating.  They renamed this gene AFB1 (a-Factor Barrier) since it seems to interfere with a-factor secretion.

The way they found this gene was by creating, as they termed them, transvestite yeast that “pretended” to be the opposite mating type. One strain that they named the MATα-playing-a strain was α but produced a-specific mating proteins, while the other, the MATa-playing-α strain, was a but produced α-specific mating proteins.  Sounds easy but it took a bit of genetic engineering to pull off.

The first steps in making the MATa-playing-α strain were to replace STE2 with STE3, MFA1 with MFα1, and MFA2 with MFα2.  In addition, they had to delete BAR1 to keep it from chewing up any α factor that got made, and ASG7, which inhibits signaling from STE3.  This strain still had the MATa locus, which meant that except for the manipulated genes, it still maintained an a-specific gene expression pattern.

Making the MATα-playing-a strain wasn’t much simpler.  They had to replace STE3 with STE2, MFα1 with MFA1, and MFα2 with MFA2.  In addition, they drove expression of BAR1 with the haploid specific FUS1 promoter and expression of the a-factor transporter STE6 with the MFα1 promoter.  Maybe yeast isn’t so simple after all!

When Huberman and Murray mated the two transvestite strains to each other, they found that while these strains could produce diploid offspring, they weren’t very good at it.  In fact, they were about 700-fold worse than true a and α strains!  So what’s wrong?

To tease this out the researchers mated each transvestite to a wild type strain.  They found that when they mated a wild type a strain to a MATa-playing-α strain, the transvestite’s mating efficiency was only down about three fold.  By overexpressing α factor they quickly found that the transvestite strain’s major problem was that it simply didn’t make enough α pheromone.  They hypothesized that perhaps differences in promoter strength or in the translation or processing of α-factor were to blame. 

The reason for the low mating efficiency of the MATα-playing-a strain, however, wasn’t so simple.  When Huberman and Murray mated the MATα-playing-a strain with an α cell, they found it was about 60-fold worse at mating.  The first thing they looked for was how much a-factor this strain was producing.  Because a-factor is difficult to assay biochemically, they used a novel bioassay instead and found that it secreted much less a-factor than did the wild type a strain.  Further investigation showed that the transvestite strain produced something that blocked the ability of a-factor to be secreted. 

By comparing the transcriptomes of MATa and MATα-playing-a cells they were able to identify YLR040C as their potential a-factor blocker.  They went on to show that when this gene was present, a-factor secretion was indeed inhibited.  They hypothesize that their newly named AFB1 may produce a protein that binds to and sequesters a-factor. It may be to a cells what BAR1 is to α cells, helping the yeast cell to sense the pheromone gradient and choose a mating partner.

When Huberman and Murray knocked AFB1 out of the MATα-playing-a strain, it now mated with a wild type α strain about five fold better than before.  A nice increase, but it doesn’t completely correct the 60-fold reduction in this transvestite’s mating efficiency.  Something else must be going on.

That something appears to be that the strain only arrests for a short time when it encounters α-factor.  This would definitely impact mating efficiency, as it is very important that when a and α strains fuse they both be in the same part of the cell cycle.  Pheromones usually stop the cell cycle in its tracks, but α-factor can’t seem to keep the MATα-playing-a cell arrested for very long.  The researchers looked for genes involved in this transient arrest, but were not able to find any one gene that was responsible.

From all of this the authors conclude that there is a pheromone arms race raging in the yeast world.  The most attractive yeast are those that make the most pheromone, so evolution favors higher and higher pheromone production.  Just as people on the dating scene need to see past the makeup and trendy clothes to figure out who’s really the best partner, yeast need genes like BAR1 and AFB1 to parse out who is the best mate amid the ever increasing haze of pheromones.