Rewiring Cellular Power Plants

[SOFIA] Every cell you have is running on a fossil. Not a metaphor — an actual ancient bacterium that moved in a couple billion years ago and never left. We call it the mitochondrion, and it's still carrying its own little scrap of genome, separate from the one in your nucleus. Today the whole show is about those power plants — how life makes energy, and how we're finally learning to engineer the machinery.

[DANIEL] And it's not just mitochondria. Chloroplasts, the fermentation enzymes in bacteria — anything that moves electrons to make ATP. That's the theme. Bioenergetics, and the tools to rewire it.

[SOFIA] Right, so let me set the table, because this connects things that don't usually sit together. The endosymbiotic organelles — mitochondria and chloroplasts — both descend from free-living bacteria that got engulfed. And here's the quirk that makes them so hard to engineer: they kept their own DNA. A tiny genome, physically walled off inside a double membrane.

[DANIEL] Which is the whole problem. For nuclear genes we've had CRISPR for over a decade. You make a guide RNA, it finds the target, the nuclease cuts. Standard. But that guide RNA has to get inside the organelle, and mitochondria don't import RNA — at least mammalian ones don't, not efficiently. The membranes have transporters tuned for proteins and metabolites, not for a piece of guide RNA you designed.

[SOFIA] So for decades, editing mitochondrial DNA was basically off-limits. And that matters clinically, because mtDNA mutations cause real, devastating inherited disease — Leigh syndrome, MELAS, the ones that hit tissues starved for energy. Brain, muscle, heart.

[DANIEL] And there's a wrinkle that makes it worse: heteroplasmy. You don't have one copy of the mitochondrial genome — you have hundreds or thousands per cell. Disease shows up when the fraction carrying the mutation crosses a threshold. So you're not fixing one target, you're trying to shift a population.

[SOFIA] Okay, so that's the setup. Let's trace how the field cracked it. The first turning point is a 2022 review from Silva-Pinheiro and Minczuk, and the key innovation they're framing is this thing called DdCBE.

[DANIEL] Which is clever precisely because it sidesteps the RNA import problem. DdCBE is a base editor built from a bacterial toxin — DddA — that can deaminate cytosine inside double-stranded DNA. And critically, it's all protein. No guide RNA. You fuse it to TALE proteins that recognize the DNA sequence, and proteins mitochondria already know how to import.

[SOFIA] That's the move that broke the logjam. Don't fight the membrane — bring a tool it'll let in. And you get a base edit, a C-to-T change, without cutting the genome.

[DANIEL] Which for mitochondria you want to avoid. If you make a double-strand break in mtDNA, the cell tends to just degrade that copy rather than repair it. So a nick-free, cut-free chemical conversion fits the biology.

[SOFIA] Then a 2024 review — the five-class survey — takes stock of the whole toolbox and lands on the same conclusion. DdCBE plus adenoviral delivery as the most credible route for heteroplasmic disease. So the field's converging.

[DANIEL] Converging, and honest about limits. Base editors do one kind of change. You can't do everything with a C-to-T conversion. Which is why the 2025 paper is such an interesting swing.

[SOFIA] Oh, this one I love. This is IM83. Because it goes back and asks the forbidden question — what if we could just get the guide RNA in after all?

[DANIEL] In yeast, to be clear. Saccharomyces mitochondria.

[SOFIA] In yeast, yes. And the trick is gorgeous. They fuse an RNA aptamer — a folded RNA sequence, this IM83 — onto the CRISPR guide. And that aptamer hijacks a native import pathway. There's a system, Msk1 feeding into the TOM-TIM translocons, that yeast mitochondria use to bring certain RNAs across both membranes. The aptamer basically forges a shipping label.

[DANIEL] So it's the mirror image of the DdCBE strategy. DdCBE said avoid RNA import entirely. IM83 says, no, let's smuggle the RNA through a door that already exists. Both are legitimate answers to the same barrier.

[SOFIA] And it worked — first sgRNA-mediated editing of mitochondrial DNA. That's the headline. Guide RNA, across two membranes, into the organelle, editing.

[DANIEL] It worked, and I want to be precise about how well, because the paper is. The efficiency is roughly four edits per hundred million cells.

[SOFIA] Which sounds tiny.

[DANIEL] It is tiny. But the honest framing is: the bottleneck isn't the guide anymore. It's that the base editor they co-deliver is toxic when imported — the import itself stresses the cell. So the guide RNA delivery is a genuine proof of concept, and the ceiling is now a different problem. That's a clean result. I trust a paper that tells me exactly where it's stuck.

[SOFIA] And that's the through-line for the organelle story — every step is really a delivery story. How do you get your tool across membranes that evolved to keep things out.

[DANIEL] Now, you promised me this show wasn't only mitochondria.

[SOFIA] I did! Because the same theme — engineering bioenergetics — is playing out in wildly different systems, and two 2024 papers make that vivid. First, the one that made me laugh out loud: Aoki and colleagues put chloroplasts inside animal cells.

[DANIEL] Red algal chloroplasts. Cyanidioschyzon merolae — a thermophilic red alga, small genome, 243 genes in that chloroplast.

[SOFIA] Into CHO-K1 cells — hamster ovary cells, the workhorse of biotech. And the chloroplast keeps running photosystem II, keeps moving electrons, for at least 48 hours. First confirmed photosynthesis inside an animal cell.

[DANIEL] For forty-eight hours. I'd want to know how they're measuring active electron transport versus a chloroplast that's just intact but idling — but PSII electron transport is a real functional readout, not just "the thing is still there." It's a recapitulation, in miniature, of the endosymbiosis that made us. Someone re-running the origin event in a dish.

[SOFIA] It's the whole arc of this show in one experiment. And then the fourth paper is the least flashy but maybe the deepest — Hackmann's 2024 survey of fermentation across eight thousand-plus prokaryotes.

[DANIEL] This is the one I keep thinking about. Over a quarter of prokaryotes ferment, using 55 end products across 123 glucose-specific reactions. And the finding that reframes it is electron bifurcation — flavin-based enzymes that split an electron pair, sending one to a hard, uphill reaction and one downhill, to balance the books.

[SOFIA] That's the hidden redox backbone. It's how all this metabolic diversity stays energetically balanced.

[DANIEL] And it ties straight to the applied paper — the Acidithiobacillus work. They built the first dCas12a CRISPRi system in an acidophile and knocked down the bc1 complex in the sulfur pathway. Reroute the electrons toward iron oxidation, and copper extraction from chalcopyrite jumps to 68%. That's electron-pathway rewiring for mining.

[SOFIA] So across every one of these — the yeast, the hamster cells, the mining microbe — the game is the same. Control where the electrons go, and control the genome that runs the machine.

[DANIEL] And the honest state of it: we can smuggle proteins in reliably, RNA barely, and we can rewire electron flow in a bacterium far more easily than in our own organelles.

[SOFIA] Which tells you exactly where it's heading. Better delivery, less toxic editors, and someday shifting heteroplasmy in a patient. The power plants are finally getting a control panel. Daniel, thank you — after the break, a single paper worth slowing down for.