New & Noteworthy

Yeast, Smarter than a Train Wreck

January 30, 2014

Imagine you run a railroad that has a single track. You need for trains to run in both directions to get your cargo where it needs to go.

One way to regulate this might be to have the trains just go whenever and count on collisions as a way to regulate traffic. Talk about a poor business model! Odds are your company would quickly go bankrupt.

Another, more sane possibility is to somehow keep the trains from running into each other. Maybe you schedule them so their paths never cross. Or maybe you have small detours where a train can wait while the other passes. Anything is better than regulation by wreckage!

Turns out that at least in some cases, nature is a better business person than many people previously thought. Instead of trains on a track, nature needs to deal with nearby genes that point towards one another, so-called convergent genes. If both genes are expressed, then the RNA polymerases will barrel towards one another and could collide.

A new study in PLoS Genetics by Wang and coworkers shows just how big a deal this issue is for our favorite yeast Saccharomyces cerevisiae. An analysis of this yeast’s genome showed that not only did 20% of its genes fit the convergent definition but that in many cases, each gene in a pair influenced the expression of the other gene. Their expression was negatively correlated: when one of the pair was turned up, the other went down, and vice versa.

One way these genes might regulate one another is the collision model. When expression of one gene is turned up and a lot of RNA polymerases are barreling down the tracks, they would crash into and derail any polymerases coming from the opposite direction. A prediction of this model is that orientation and location matter.  In other words, the negative regulation would work only in cis, not in trans.  Surprisingly, the authors show that this is clearly not the case.

Focusing on four different gene pairs, Wang and coworkers showed that if the genes in a pair were physically separated from one another, their expression was still negatively correlated.  This was true if they just flipped one of the genes so the two genes were pointed in the same direction, and it was still true if they moved one gene to a different chromosome.  Clearly, collisions were not the only way these genes regulated one another.

Using missense and deletion mutation analysis, the authors showed that neither the proteins from these genes nor the coding sequence itself was required for this regulation.  Instead, the key player was the overlapping 3’ untranslated regions (UTRs) of the transcripts.  The authors hypothesize that the regulation is happening via an anti-sense mechanism using the complementary portions of the 3’ UTRs.

This anti-sense mechanism may be S. cerevisiae’s answer to RNAi, which it lost at some point in its evolutionary history.  Given the importance of RNA-mediated regulation of gene expression in other organisms, perhaps it shouldn’t be surprising that yeast has come up with another way to use RNA.  

Instead of RNAi, it relies on genomic structure and overlapping 3’ UTRs to regulate genes.  This may be a bit more cumbersome than RNAi, but at least yeast came up with a more clever system than polymerase collisions to regulate gene expression.  

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: RNA polymerase II, Saccharomyces cerevisiae, transcription, UTR

More Going on in the Ribosome Than Expected

January 20, 2012

As scientists peer ever more deeply into a cell, the picture of how things work becomes more and more complicated.  This was true when scientists took a hard look at transcription and gene regulation and found lots of little RNAs scurrying around the cell, regulating genes.  And it now appears to be true for what is being translated and how translation is regulated.

In a new study, Brar and coworkers used ribosome profiling to explore what happens in yeast cells during meiosis at the level of translation.  What they found was that a whole lot more was being translated (or at the very least gumming up the translation machinery) than anyone expected. They also found that translation is as finely regulated as is transcription. 

And this doesn’t just happen in yeast.  The same group has also generated similar findings in mice embryos as well.  Results with human cells should be right around the corner…

Ribosome Profiling

In ribosome profiling, scientists determine what RNAs are contained in a ribosome at a given time point.  The basic idea is that they isolate ribosomes, treat them with nucleases and then harvest the associated 30-35 nucleotide long mRNAs.  They then sequence all of the isolated RNAs and identify where they came from.

Like lots of biology these days, this technique has only become possible with the advent of cheap, robust sequencing.  In fact, the size of these sequences is ideal for modern sequencing techniques.

Researchers in the Weissman lab are finding all sorts of interesting things using this new tool.  For example, in meiosis they were better able to determine which proteins are involved at various stages of meiosis, to see how involved “untranslated” mRNA leaders are in translation, and to identify smaller, previously ignored transcripts associated with ribosomes.  In this post we’ll just focus on the last point but encourage the reader to learn about the study’s other findings here.

Of Shorter ORFs

Ribosome profiling has revealed that a lot more is being translated in yeast than the standard set of genes identified in the Saccharomyces Genome Database (SGD).  For example, Brar and coworkers found that the mRNA of many open reading frames (ORFs) shorter than the usual 250 or so base pairs were associated with the ribosomes.  Shorter ORFs like these aren’t routinely thought of as genes and so have not been extensively studied.

However, given how many of these ORFs were associated with ribosomes, scientists probably should start paying more attention now.  Even before meiosis, 5% of the ribosomes tested in yeast contained RNAs from these shorter ORFs.  Once meiosis kicked in, the number went up to an astonishing 30%.

Since scientists have only just started to focus on them, it isn’t surprising they don’t know how many of these smaller ORFs are translated into smaller peptides.  Or what any of these peptides that do get translated might be doing in a cell.

In a recent study, Kondo and coworkers have shown that one of these ORFs is translated into a peptide and proposed it affects how the transcription regulator Shavenbaby works in Drosophila.  Work similar to this will need to get underway before we have a good handle on what exactly is going on with these shorter ORFs.

Whatever they turn out to do, these small ORFs will probably change what we consider to be a gene.  Again.  The cell just keeps getting more and more complicated! 

Lengthy but informative lecture on ribosome profiling.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: regulation, ribosome, ribosome profiling, translation, untranslated leader, UTR

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