Reconstituted Saccharomyces cerevisiae mismatch repair shows the Mlh1–Pms1 endonuclease directly generates single-strand gaps to excise mispairs independent of Exo1 and Rad27.
0:00Welcome to Base by Base, the papercast that brings genomics to you, wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. It's fantastic to be back. We have um, a real biochemical detective story on our hands today.
0:13We really do. And to kick things off, I want to step away from the lab bench for a moment. I want you to imagine you are running a, a high-end, very specialized auto repair shop. Okay. You have a strict division of labor in this garage.
0:29You have one mechanic. He only touches engines, you have another, she only touches tires. Got it. Efficient. The classic assembly line model. Everyone stays in their lane. Exactly. But one day, you decide to run a sort of chaotic experiment.
0:44You fire the engine guy. And just to be safe, you fire the tire guy too. You remove the specialists entirely. I assume the whole shop just grinds to a halt cars start piling up. That's exactly what you would expect, but strangely, no.
0:55The cars keep rolling out of the garage, fully repaired. The engines are purring, tires are balanced, business as usual. That is, well, it's unsettling. It implies someone else is doing the work when you aren't looking.
1:07Right. If the mechanics are gone, who is turning the wrench? For decades, this was the exact paradox facing scientists who studied DNA mismatch repair. We thought we had the crew list. We thought we knew exactly which enzymes were the mechanics responsible for you know, cutting out errors in our DNA.
1:25And when those enzymes were removed in experiments, the system, just didn't crash. It didn't crash. It barely even slowed down. So today we're doing a deep dive into the hidden hands of DNA repair. We are going to find out how the character we thought was just the manager.
1:40Everyone just shouting orders. Might actually be the one rolling up its sleeves and doing the dirty work itself. This is a major, major shift in how we understand genomic stability. We are looking at the article titled DNA Mismatch Repair, mediated by MLH1, PMS1, and a nucleus catalyzed Miss Bear Excision.
1:58A bit of a mouthful, but we'll break it down base by base. We will. And before we get into the weeds, we should recognize the team behind this, this is a massive effort. The research is led by Tatiana Palacio and Felipe A. Khalil, along with Richard D.
2:12Kaladner and their colleagues. And this is all coming out of UCSD, the University of California, San Diego School of Medicine. A real powerhouse in this field. They are really pushing the boundaries of what we know about, you know, DNA maintenance.
2:24So let's start with the basics. We mentioned DNA mismatch repair or MMR. I usually describe this to friends as the spell checker for our genetic code. Is that, uh, am I oversimplifying? No, that is the perfect analogy?
2:37Think about it. When your DNA replicates, it's copying 1000000000s of letters. A, with T, C, with G. But the copier, the polymerase, it's fast and it makes mistakes. Like typing a text message too quickly.
2:48Exactly. Maybe it puts a C where an A should be? That's the mismap. The type. Right. And if that tipo stays there, it becomes a permanent mutation the next time the cell divides and mutations. Well, they can drive cancer, genetic diseases.
3:01So MMR is the system that scans the fresh DNA, spots that typo and fixes it before the ink is dry. Okay, so that's the goal. Fix the typo. Now walk me through the old world view. Before this new paper.
3:14How did we think the spell checker worked? The textbook model, the one I learned, the one still in many textbooks, is the relay race. First, you have the recognition team. These are proteins called MS2 and MS 6.
3:25They are the proofreaders. They slide along the DNA and stop when they feel a bump. A distortion caused by the mismatch. So proofreaders find the error, they flag it, then what? Then they recruit the scissors.
3:37In the classical view, these are specialized enzymes called exonucleses. The big celebrity here is one called XO1, XO1, its job is to bind to the DNA and just chop out a whole section of the strand with the error.
3:51It unzips it, basically. And once that bad chunk is gone. Polymerase comes in and fills the gap with the right letters. Find, cut paste. So, find. Cut paste. And XO1 does the cutting. That was the dogma.
4:04XO1 is a primary cutter. There's a backup called Rad 27, but XO one is the star. But this is where the story gets weird where that auto shop analogy comes back. You said the system didn't crash when they removed these tools.
4:15How strong is that evidence? Oh, it's overwhelmingly strong. And baffling. Years ago, researchers deleted XO1 in yeast cells. They expected the mutation rate to just skyrocket. Because the repair system should be broken.
4:29It should be, but it didn't. They saw less than a one% defect compared to a totally broken system. Wait, wait, one%. So 99% of the repairs were still happening without the main pair of scissors. Correct.
4:41And even when they got rid of both XO one and the backup, red 27, most of the repair was still happening. That's wild. That's like removing the engine from a car and it still does 0 to 60 in 4 seconds.
4:51Exactly. The field knew there was this this dark matter in DNA repair. They called it the XO1 and RAD 27 independent pathway. We knew that it happened, but we had no idea how. Who was cutting the DNA? Who was cutting the DNA?
5:04And that brings us to this paper. They went hunting for that ghost mechanic. How do you even begin to look for something like that. You can't just stare at cells under a microscope. No, living cells are way too messy.
5:15Thousands of proteins all buzzing around. If you want to find the hidden mechanic, you have to build the garage from scratch. Reconstitution. Yes. They used a reconstituted system. They purify the individual proteins, MSTO, MLH1, all parts, and mix them in a test tube.
5:31So they built a biological engine on a workbench. Pretty much. They took circular loops of DNA, classmids, and engineered them to have a specific typo, like a CC Ms. Bear, and they also put a little nick in the DNA nearby to simulate a replicating strand.
5:46So they have the broken DNA. They have the repair crew. But they left out XO one and RAD 27. Deliberately. They withheld the known scissors. They wanted to see if the remaining proteins could fix the error on their own.
5:56And they had their eyes on one specific suspect, a complex called MLH1 PMS1. Now, I've heard of MLH1 PMS1. In the old model, isn't this just the matchmaker, the middleman. That was the assumption, yeah.
6:10We thought it just coordinated things. It would come in, maybe make a tiny nick to mark the spot and then call XO1 to do the heavy lifting. But the researchers suspected the middleman might be hiding a weapon.
6:21They suspected it might be doing the cutting itself, but proving that is incredibly difficult. You have to see exactly what's happening to the DNA molecule. You need to see the cuts, the gaps. The actual physical changes.
6:34And this is where the paper gets really creative. I mean, they use standard methods, sure, but then they use something called the Abobec 3A trick. Oh, this is clever. This is a highlight for listeners who love the biochemical details.
6:46It's so clever Break it down for us. What is apobix 3A? It sounds like a droid from Star Wars, right? Apobic 3A is an enzyme that attacks DNA, but, and here's the key. It only attacks single stranded DNA.
7:00Okay, why is that useful here? Well, DNA is usually a double helix, zipped together. But when an enzyme repairs a mistake by excision, it has to chew away one strand. Creating a gap. Exactly. That gap leaves the other strand exposed.
7:14It's single stranded because its partner has been eaten. And that's when apobic strikes. Yes. They threw Apobic into the mix. So if MLH1 PMS1 was creating gaps. Apobuck would attack that exposed strand and chemically change the letters.
7:29It turns cytocenes into urosils. It's like dusting for fingerprints. It is exactly like that. Or footprints in wet cement. By sequencing the DNA later, they could look for those changes. Wherever they found them, they knew.
7:41Aha. The DNA was single stranded here. The enzyme chewed this part up. That is unbelievably precise. Usually you just see repaired or not repaired. This tells you how. It visualizes the invisible. It maps the repair down to the single letter.
7:54So they have the test tube engine, a fingerprint powder. What'd they find? Was the manager, MLH1 camas one actually fixing the car? The manager is a master mechanic. The study proved unequivocally that MLH1 PMS1 can drive the repair entirely on its own.
8:09It does not need XO1. It does not need RAD 27. So it's not just pointing fingers and coordinating. It's holding a knife. I like to call it the wood chipper. The wood chipper. That sounds aggressive for a biological process.
8:21It is aggressive. Compared to XO1, XO1 is like a zipper. It starts at a cut and just unzips in a continuous line. It's very orderly. One long segment. One long segment. MLH one dash PMS one. It's messy.
8:33It makes multiple cuts, it chops. The electron microscopy where they took literal pictures of the DNA, showed these gaps being formed by distinct chopping actions. It essentially shreds the DNA around the air.
8:45So it turns the bad DNA into confetti. effectively. And here's a crucial detail that explains why this was missed for so long. This wood chipper mode, it has a specific fuel source. A fuel source. What do you mean?
8:57Well, enzymes often need metal ions to work. Co-factors. XO1, the classic enzyme, loves magnesium. Most lab buckers are made with magnesium. Okay, so that's the standard. But MLH1, PMS1, to go into this beast mode, it prefers manganese or zinc.
9:12Manganese. Yes. So if you're only looking at these reactions in magnesium, which is standard. You see a little bit of activity, but you might miss just how powerful it really is. With manganese present, it becomes a robust excision machine.
9:24That explains why it might have been hidden in other experiments. We weren't giving it the right fuel. Precisely. It was there all along, but we were feeding it the wrong diet. I want to go back to the wood chipper visual.
9:36When they looked at the gaps, Were they just random? Does it just shred the whole strand? Because that sounds dangerous. No, and that's the beauty of it. It's controlled chaos. The mapping revealed 2 specific types of gaps.
9:50What are they? First, you have what they call polar gaps. These are long 100s of letters long. They start at that initial nick in the DNA, and they extend toward the error. Okay, so it starts with the cut and eats its way down, directional.
10:04That makes sense. But then they found disperse gaps, and these were fascinating. Short, scattered gaps right around the air itself, sometimes not even connected to the main cut. That sounds risky. If you're chopping around the air.
10:17Aren't you worried about chopping the wrong strand, the template? That is the number one rule of DNA repair. Do not touch the template. If you alter the master copy, you aren't fixing an error, you're cementing it.
10:28You're creating a mutation. So how does the woodchipper know which strand is shred? It has strict strand specificity. It only targets the strand that is already nicked, the new strand. The study showed it.
10:39The gaps were exclusively on that broken strand. The template was pristine. That is just such a sophisticated system. It's aggressive, but it's precise. It knows exactly which rail of the track is broken.
10:51And it's versatile. Once MLH1 PMS1 has shredded the bad section, a polymerase has to come fill it in. They found that multiple types, pull epsilon or pull delta, can do the job. It's not picky about the cleanup crew.
11:04Exactly. As long as the ed sequence is gone, the cell has tools to patch the hole. So we've established that the manager, MLH1 PMS1, is actually this secret agent who can do the whole job solo, but I have to ask the why question.
11:17evolutionary question. Yeah. If we have XO1 and we have RAT 27 and they work, why does the cell need this 3rd way of doing things? Isn't that overkill? In biology, there is no such thing as overkill when it comes to DNA integrity.
11:30Redundancy is survival. It's the backup for the backup? Think about the stakes. If this system fails, I mean, really fails, the cell accumulates mutations that leads to cell death, or, worse, cancer. Evolution does not take chances with this.
11:45So it's like wearing a belt and suspenders and then also duct taping your pants to your waist. That is a vivid image, but yes. This 3rd pathway explains why those mice we talked about didn't get massive cancer rights.
11:59You knock out the main plan. XO1, the cell has a backup, Rad 27. You knock that out, MLH1 PMS1 steps in with the wood chipper. It creates a safety net that is almost impossible to fall through. And it solves a massive mystery in the field.
12:11Scientists always wondered, why is MLH1 PMS1 required for all mismatch repair? Right, because even in the old model, if you deleted MLH1 PMS one, the whole system just broke. Exactly. If it was just a helper for XO1, you'd think XO1 could manage without it, but it can't.
12:26This paper suggests MLH1 PMS one is the central hub. It might be doing some level of cutting and processing in every repair event. Not just as a backup. Not just as a backup. It's likely the core processor of the whole operation.
12:37So the authors propose a new model, right? The 3 pathway model. You do. a hierarchy. Pathway one, XO one dependent, the classic zipper, pathway two, RAD 27 independent, a bit different. And then pathway three, this MLH1 PMS1 mediated excision, the wood chipper.
12:54And all 3 work together to keep our code clean. It seems so. And we should mention, there are still things we can't see. The electron microscopy. I mean, it's amazing, but it had a resolution limit. They couldn't see gaps smaller than about 30 nucleotides.
13:09So there might be even tinier microsurgery happening that we're missing. Almost certainly. MLH1 PMS one might be making tiny nicks that didn't even show up on camera. But what we can see is robust proof that this protein is a powerhouse.
13:23It really changes the narrative. We used to look at MLA1 PMS1 and say, oh, good job coordinating. Thanks for the help. Now we realize it's the one holding the chainsaw. A very precise manganese activated chainsaw.
13:34I want to zoom out a bit. We've gone deep into the biochemistry, the enzymes, the manganese. For the listener at the gym right now. Why does this matter? What's the big takeaway? The core insight? Is that our view of biological machinery is often way too simple?
13:47We like to put proteins in boxes. Yeah. This one cuts, this one signals. This paper proves that nature is far more versatile. MLH1 PMS1 is a robust, standalone machine that guarantees that even if the specialists fail, the jod gets done.
14:01It speaks to the resilience of life. We are built to survive our own errors. We are, and it creates a really fascinating prompt for us to consider, you know, when it comes to medicine. Well, think about cancer treatment.
14:13Cancer cells are often defined by their instability. Their DNA repair is often broken or rewired. That's why they mutate so fast. Right. So if we know that healthy cells have 3 distinct layers of redundancy.
14:25But a cancer cell might be relying heavily on just one of them because the other 2 are broken. Ah, we could target that specific pathway. Exactly. This is the concept of synthetic lethality. If we understand the backup mechanics, we can find ways to close the garage on cancer cells permanently.
14:40If we know a tumor is surviving solely because of the MLH one PM's one pathway, and we create a drug to block that wood chipper. The cancer cell can't repair itself, and it dies. While the healthy cells, which still have their other pathways might be totally fine, it opens up new doors for precision medicine, using the cancer's own weaknesses against it.
15:00That's a powerful thought. It takes this from abstract biochemistry to something that could actually save lives. Absolutely. The more we know about the parts list, the better we can tune the engine. Well, that is where we will have to leave it for this deep dive.
15:14A dense paper but a really, really important one. A huge thank you to the team at UCSD for shedding light on these hidden hands. It was a pleasure unpacking this mystery with you. This episode was based on an open access article under the CCBY 4.0 license.
15:29You can find a direct link to the paper and the license in our episode description. If you enjoyed this, follow or subscribe in your podcast app and leave a 5 star rating. If you'd like to support our work, use the donation link in the description.
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