December 10, 2015
As the holidays approach, many of us are getting ready to crowd around the table for a big family dinner. Some of us may behave differently around family than we might with friends or coworkers.
For example, with your relatives, you might bite your tongue if your political views vary greatly from theirs. Where we are and with whom we interact can sometimes affect what we do.
Turns out that yeast growing in a colony can be the same way. Though of course they aren’t keeping their opinions to themselves. (Well, we don’t think they are…)
A yeast cell can end up acting differently depending on where it is in a colony. For example, only a narrow band of cells gets to sporulate while all the others are left to plod through mitosis.
A new study out in GENETICS by Piccirillo and coworkers shows that these cells sporulate because nearby cells “encourage” them to. They are being influenced to sporulate because of the cells around them. Just like your relatives might influence you to change your behavior at the dinner table.
The first step in showing that one set of cells signals a second set to sporulate was to find the genes involved in setting up this pattern. Since the authors were looking at Saccharomyces cerevisiae, it was pretty easy to get mutants to study. They just had to open their freezer and pull out their yeast homozygous diploid deletion library.
Initially, they looked for strains where the usual pattern of sporulating cells was disrupted. They then took these candidates and looked for those that could still sporulate normally in suspension. They wanted mutants that could sporulate but couldn’t do it in the right place.
They found seven strains that fit the bill. Three of the deleted genes, MPK1/SLT2, BCK1, and SMI1, were in the cell-wall integrity pathway (CWI). They also showed that mutation of three other genes in the pathway, SLG1/WSC1,TUS1 and RLM1, all impacted colony sporulation as well.
Further work showed that the transcription factor RLM1 was induced 1-2 days before the master regulator IME1 was turned on. IME1 is a key player in getting meiosis started so that yeast cells can sporulate.
So the story seemed to be that RLM1 is turned up which then turns on IME1, which kick starts meiosis. Makes sense except it is unlikely that Rlm1p is directly activating IME1. There is no obvious Rlm1p site in the IME1 promoter.
A close look at the colonies showed that RLM1 is upregulated in a layer of cells just under the ones where IME1 is upregulated. Deletions in the CWI pathway seemed to have disrupted a group of “feeder” cells whose job it is to get nearby cells to sporulate.
To show this, the authors used a chimeric colony assay that consisted of two strains. The first strain, which had functional Rlm1p, had a reporter, either RFP or lacZ, under the control of the IME1 promoter. The second strain was either wild type or deleted for the transcription factor RLM1.
They created colonies with equal amounts of each strain and looked at IME activation. The idea is that if RLM1 is important in the cells that sporulate, then the second strain shouldn’t matter. You should get the same number of cells in which the IME1 promoter is activated whether or not adjacent cells express RLM1.
But if it is important for RLM1 to be expressed in nearby cells, then there should be a falloff in activation if adjacent cells are deleted for RLM1. This is just what the authors found.
And it wasn’t just the artificial reporter system that was affected either. There was also a drop off in the number of cells that sporulated in the case where some of the cells lacked RLM1.
In a further set of experiments, Piccirillo and coworkers showed that these feeder cells became more osmosensitive compared to the ones that go on to sporulate. While they did not find the signal that prompted the meiosis of nearby cells, this change in osmosensitivity is consistent with the cells preparing to release something into the environment.
So it looks like activating the CWI pathway in one set of cells causes a second set to start down the road of sporulation. And if the CWI pathway is disabled in these cells, then the second set of cells no longer changes their behavior and begin to go through meiosis.
This all seems weird at first until you realize that the cells in a colony usually all share the same DNA. What is good for one set of cells is good for the survival of the DNA even if it is at the expense of other cells in the colony.
Yeast cells tend to sporulate when food grows scarce. But sporulating takes a lot of energy. Colonies may get around this paradox by having some of the cells in the colony give up nutrients or energy to a few cells that go on to sporulate. The feeder cells deprive themselves so that other cells have a better shot at survival.
Now the DNA, shared by all the cells, can live on for the next round of holiday dinners….
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight