New & Noteworthy
March 17, 2017
For almost 50 years, the legendary Yeast Genetics & Genomics course has been taught each summer at Cold Spring Harbor Laboratory. (OK, the name didn’t include “Genomics” in the beginning…). The list of people who have taken the course reads like a Who’s Who of yeast research, including Nobel laureates and many of today’s leading scientists.
The application deadline is April 15th, so don’t miss your chance! Find all the details and application form here.
This year’s instructors – Grant Brown, Maitreya Dunham, and Elçin Ünal – have designed a course (July 25 – August 14) that provides a comprehensive education in all things yeast, from classical genetics through up-to-the-minute genomics. Students will perform and interpret experiments, learning about things like:
- How to Find and Analyze Yeast Information Using SGD
- Isolation and Characterization of Mutants
- Transformation of Plasmids & Integrating DNAs
- Meiosis & Tetrad Dissection as well as mitotic recombination
- Synthetic Genetic Array Analysis
- Next-Gen. whole-genome and multiplexed DNA barcode sequencing
- Genome-based methods of analysis
- Visualization of yeast using light and fluorescence microscopy
- Exploring synthetic biology with CRISPR/CAS9-directed engineering of biosynthetic pathways
Scientists who aren’t part of large, well-known yeast labs are especially encouraged to apply – for example, professors and instructors who want to incorporate yeast into their undergraduate genetics classrooms; scientists who want to transition from mathematical, computational, or engineering disciplines into bench science; and researchers from small labs or institutions where it would otherwise be difficult to learn the fundamentals of yeast genetics and genomics. Significant stipends (in the 30-50% range of total fees) are available to individuals expressing a need for financial support and who are selected into the course.
Besides its scientific content, the fun and camaraderie at the course is also legendary. In between all the hard work there are late-night chats at the bar and swimming at the beach. There’s a fierce competition between students at the various CSHL courses in the Plate Race, which is a relay in which teams have to carry stacks of 40 Petri dishes (used, of course). There’s also a sailboat trip, a microscopy contest, and a mysterious “Dr. Evil” lab!
Last year’s Yeast Genetics & Genomics Course was loads of fun – don’t miss out!
March 13, 2017
A square slab can make an excellent door stop. Over time, though, the corners can get chipped, making the slab a bit rounded. This new bit of rock makes a less useful doorstop, but a much better wheel! The chipping and aging of the original stone has made it worse in some situations, but better in others.
A new study by Frenk and coworkers in Aging Cell shows that something similar can happen in yeast. Young yeast are much better at utilizing glucose, but older yeast have them beat with galactose (as well as with raffinose and acetate).
One way to think about this is that age turns yeast from a glucose specialist into a sugar generalist. Aging chips away at a yeast cell’s ability to use glucose, but this loss results in a gain in its ability to use galactose.
So at least in the right environment (i.e., when there’s lots of galactose around), with our old friend Saccharomyces cerevisiae, there can be advantages to getting older.
What makes this particularly fascinating is that at least in yeast, this suggests that there may be a positive selection for aging because of the advantage it can give in certain environments. Those yeast who are ageless would compete less well compared to their aging counterparts when their glucose was taken away. The aging process wins out over immortality!
Frenk and coworkers used a relatively simple experimental set up. Take young cells and old cells, mix them together, and see which outcompetes the other using various sugars.
They used yeast that had been aged for 6, 24, and 48 hours in glucose. This is a nice range as 6-hour “old” yeast are fully viable, 24-hour “old” yeast are starting to suffer a bit in the reproductive viability department, and the 48-hour “old” yeast have passed the median lifetime of a yeast cell. Young adult, middle aged, and elderly yeast.
In the first experiment, they compared these yeast to log-phase yeast which the authors refer to as young (vs. the other three which are referred to as aged).
While the 6 hour yeast could hold its own against the young yeast in glucose, the 24-hour and 48-hour yeast grew much more slowly. This is what you would expect, the younger yeast growing faster than the older yeast. The young guns outdoing the older generations.
The situation was different in galactose. Here, the elderly, 48-hour yeast, ran circles around the young yeast. They blew them out of the water.
And it appears to be an age thing. When they compared 6- and 48-hour yeast that were aged in galactose instead of glucose, the more aged yeast still won. So, it wasn’t the shift in environment that caused the difference, it was in the older yeast cells all along.
The change is also not permanent. The offspring of the older yeast weren’t any better at growing in galactose than the younger yeast were. Only the cells that had lived a longer life could use galactose so well.
A concern here is that yeast as old as 48 hours are pretty rare in the wild. But when they changed assays and looked at colony size as opposed to competition, they saw that even 18-hour yeast had an advantage over the young whippersnappers.
This was such a surprising result that they also looked at cell cycle times of individual aged cells and their daughters. The older mother cells cycled faster in galactose than their daughters. And the opposite was true in glucose.
So it really looks like there are advantages to growing older. Things break down a bit, but that breakdown uncovers new talents that had previously lain dormant.
If you’re a yeast, growing old is not a one-way decline into dotage. You gain new abilities that, under the right conditions, let you outcompete your children! The older cells are selected for in the right environments. #APOYG shows us something good about growing older.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
March 2, 2017
You may have heard the old wives’ tale of feed a cold, starve a fever. Turns out that this isn’t particularly good advice (although some studies do suggest that with a fever, you shouldn’t force-feed yourself). It also turns out to have probably originated in the 19th century and not from Chaucer in the 14th as many websites claim.
But while starving a fever is probably never a good idea, starving a cancer can be. Not by following the medical myth that since cancers use a lot of sugar, you can starve them by cutting down on sugar in your diet. Instead you can starve some cancers by denying them the amino acid asparagine (Asn).
On their way to becoming cancerous, acute lymphoblastic leukemia (ALL) cells lose their ability to make Asn. This means that unlike the cells around it, they need to pull Asn from the blood to make their proteins and to survive.
Doctors exploit this weakness by injecting L-asparaginase amidohydralase (L-ASNase) into patients which starves the cancer cell by depleting Asn levels in the blood. The cells around the cancer cells are fine because they can still make Asn.
Right now doctors use L-ASNase from two different bacterial sources: Escherichia coli and Erwinia chrysanthemi. But if a recent study by Costa and coworkers in Scientific Reports holds up, they might want to think about switching to using the Saccharomyces cerevisiae L-ASNase encoded by the ASP1 gene.
An older study had suggested that the yeast enzyme might be too weak to be useful. This new study finds that this is not the case.
The difference between the older study and this one was the purification protocol. The older study purified the native enzyme through multiple chromatography steps while this study used a single affinity chromatography step. The purified yeast and E. coli versions have comparable activity in this study.
They are also comparable in terms of being able to work with very low concentrations of Asn. This is important as Asn levels are very low in the blood.
What makes the yeast enzyme potentially better is that it is much worse at hydrolyzing a second amino acid, glutamine, than are the bacterial versions. This higher specificity for Asn is important because one of the major side effects of the current treatment is neurotoxicity caused by decreased levels of glutamine in the blood. Since the yeast version hydrolyzes glutamine at a lower rate, they predict patients may not suffer as badly from this side effect with the yeast version.
Of course this is all for naught if the yeast enzyme can’t kill cancer cells! Or if it kills cells indiscriminately.
The S. cerevisiae version was nearly as good as the E.coli version in tissue culture. After 72 hours of incubation, both versions had little effect on normal cells (HUVEC), and both were cytotoxic to the L-ASNase-sensitive cell line MOLT-4 with the E. coli version killing 95% of MOLT-4 cells and the yeast version killing 85% of them.
Taken together these results suggest that the S. cerevisiae version may be an alternative to the bacterial versions. It may be able to kill cancer cells with fewer side effects.
But the yeast version is not the only alternative in town. Another group is engineering the E. coli version to lessen its propensity for hydrolyzing glutamine. Either way it looks like certain leukemia patients may be getting an effective cancer treatment with fewer side effects.
Beer, wine, bread, chocolate, and now maybe a treatment for a nasty form of leukemia. Yeast may be humanity’s best friend. #APOYG!
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
February 15, 2017
If you have a legal problem, you get a lawyer. A medical problem, a doctor. A leaky faucet, a plumber.
And if you are trying to find and figure out if a protein is a prion, you turn to the model organism where it is best understood. Yes, that is our old friend, the yeast Saccharomyces cerevisiae.
Prions became famous in the 1980’s when mad cow disease started to pop up in Britain. These are fascinating proteins that can cause an inheritable disease without affecting a cell’s DNA.
What happens is that prions can undergo a spontaneous conformational change. Now of course, this isn’t all that special. Lots of proteins can exist in different conformations.
But what makes a prion special is that the spontaneous change sets off a chain reaction where pretty much every copy of that prion protein is converted to that second conformation. And every translated prion protein thereafter both in the cell and any cells derived from that cell have that conformation too. So the new conformation along with the new traits it confers is passed on stably.
In mad cow disease, this spontaneous change causes neurological problems eventually resulting in death. But not every prion is so dangerous. Sometimes, as is the case in yeast, they can give new properties that allow survival in a new environment. Now these yeast have a new advantage in the absence of changed DNA.
They found that some bacteria have proteins that can and do behave as prions in a laboratory setting. The next step is to determine if they ever do so in the wild. If they do, then this would tell us that functioning prions may have evolved before the Bacteria/Eukaryota split and so be older than scientists previously thought.
The first step in finding a bacterial prion involved combing through a few bacterial genomes. Did I say a few? I meant something like 60,000 of them!
Of course you need the right tool for your search. This is where yeast’s incredibly well characterized set of prions comes in handy.
Yuan and Hochschild used an algorithm trained on known yeast prions to search through the bacterial genomes for proteins that have domains that would be predicted to be able to enter into a prion conformation. Among the proteins they found was the Rho protein in Clostridium botulinum E3 strain Alaska E43. Like the authors, I’ll call this protein Cb-Rho from here on out.
As you might remember, Rho is that famous transcription termination protein found in many different bacteria including E. coli. The E. coli version, however, does not look particularly like a prion nor did the authors find that it acts like one either.
A hallmark of prions is that one of the conformations involves their ability to form amyloid aggregates. These authors used a bacterial assay to show that this happened with Cb-Rho.
They next checked whether the prion portion of the protein behaved as a prion in a yeast cell. They substituted the prion domain from Cb-Rho with that of the one from Sup35p, a yeast prion. This chimeric protein behaved similarly to wild type Sup35p.
They next tested their protein in E. coli. Often the prion conformation of the prion protein results in decreased activity of the protein. This is true in the Sup35p case, for example.
Sup35 acts as a translation release factor – it is an important factor in stopping translation at a stop codon. It is less active in its prion form meaning that there is now more read through of stop codons.
Rho is a transcription termination factor and so it would make sense that the prion conformation of Rho would result in lower levels of transcription termination. This is just what the authors found when they tested Cb-Rho in E. coli.
They used a reporter in which a transcription termination site was placed upstream of the lacZ gene. The idea is that if transcription termination is compromised, more lacZ will get made making the colonies a darker blue. And since termination is not 100% efficient, if Rho is doing its job, the colonies will be light blue.
When they plated out E. coli containing an engineered form of Cb-Rho, they got two classes of colonies, light blue and dark blue. And more importantly, this colony color was inheritable. In other words, light blue colonies gave more light blue colonies and dark blue colonies gave more dark blue colonies.
This isn’t to say that the colony color was a permanent state. It wasn’t. A bit under 1% of the colonies would spontaneously revert to the other form, as you’d expect from a prion protein.
So with the invaluable help of yeast, these authors were able to find and validate a protein from bacteria that can act as a prion. If they find a function for this in the wild, then yeast will have helped us better understand the evolution of life on Earth. Again. #APOYG!
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics