The enzyme RNA polymerase (RNAP) carries out transcription, copying DNA into RNA. It's the first step in gene expression, and a process fundamental to all life. But the inner workings of this essential enzyme have long baffled scientists. Trying to work out how it performs its core chemical reaction, which stacks new RNA building blocks one nucleotide at a time, has proven especially difficult.
Now, for the first time, scientists have captured RNAP at the fleeting moment just before this reaction occurs—after substrate binding but before catalysis initiates. Using cryo–electron microscopy, the team observed the enzyme in action, and the resulting structures, published in Molecular Cell, show that RNAP drives the process through precise spatial alignment and a coordinated chain of water molecules that shuttle protons.
Because this architecture is nearly identical across all domains of life, the findings establish a universal blueprint for how RNA synthesis is catalysed. The results also provide a framework for explaining how certain mutations disrupt the enzyme.
"This is the first time that anyone has solved these structures during an ongoing reaction," says Andreas Mueller, research associate in Seth Darst's Laboratory of Molecular Biophysics. "We now have the closest structural snapshot to date of the transition state."
Bringing gene transcription into focus
The basic steps of transcription have long been understood. RNAP builds an RNA chain one nucleotide at a time, forming a new bond between the growing chain and each incoming building block. This reaction depends on two magnesium ions, one that helps prepare the RNA for the reaction and another that stabilizes the byproducts. A flexible part of the enzyme, known as the trigger loop, folds into place to position everything correctly and speed the process along.
But one key step remained unresolved: how the enzyme removes a proton to kickstart the reaction. Researchers debated whether this was done by the enzyme itself or by nearby water molecules acting as proton carriers, with competing models proposing different mechanisms.
Clarity was hard to come by because visualizing the enzyme at the exact moment just before the reaction occurs is inherently difficult—the reaction happens too quickly to capture. Scientists initially produced a promising approximation by using X-ray crystallography, a technique that determines structure by locking molecules into rigid crystals, and slightly altered building blocks to stop the reaction and hold the enzyme in place.
Without any means for observing the reaction's true transition state, these modified building blocks served as the next best option. But these substitutions forced the enzyme into artificial states, altering its natural geometry. The resulting structures failed to capture how the enzyme actually operates.
The advent of cryo-electron microscopy, which captures molecules frozen in a near-natural state, marked a turning point. As the technology matured, Darst and colleagues realized it could be used to capture the enzyme in action at near-atomic resolution and resolve the question of how RNAP initiates its core reaction.
The chain reaction
For the study, the team paused the enzyme without altering it by supplying E. coli RNAP with three RNA building blocks while withholding the fourth. This forced the enzyme into a loop, producing short RNA fragments and restarting the process again and again. The loop, known as abortive transcription, enriched the sample with enzymes in their earliest, catalytically active stages. The team then flash-froze the reaction by plunging it into liquid ethane, locking the molecules in place in action. Using cryo-EM, they imaged about 2 million individual particles and computationally sorted them into distinct structural states, building near-atomic models detailed enough to resolve ions and even individual water molecules.
A series of snapshots along the reaction pathway emerged, including images of the long-sought moment just before a new bond forms. In the pre-reaction state, the team found that the enzyme folds tightly around its substrate, positioning the RNA chain and incoming building block in near-perfect alignment so that the reaction can occur.
The structures also clarified how RNAP gets the reaction started, resolving the long-standing debate. The researchers observed a continuous chain of water molecules extending from the active site, where the reaction occurs, to the surrounding solution. This water network provides a path for protons to exit, supporting a water-mediated mechanism. Additional structures show the enzyme reopening after the reaction, releasing its chemical byproducts, and preparing to move forward along the DNA. The same water-based architecture appears in related enzymes from yeast, suggesting that this mechanism may be conserved across other domains of life.
Together, the findings establish the first definitive blueprint for how transcription begins. The next step is to capture the enzyme in action with the other possible DNA-RNA base pair combinations, to see how it adapts to different building blocks while maintaining accuracy. "The genetic code consists of four bases," Mueller says. "Now that we've captured how the enzyme looks, in action, with one base, it would be really interesting to compare this to the other three bases, since they differ in size and chemistry and may change how the active site is arranged."
But even before that, the results redefine how this essential enzyme works at its most fundamental level. And because the active site of RNAP is nearly identical across all forms of life, the implications extend far beyond E. coli. "This is the enzyme that transcribes DNA into RNA in all bacterial, archaeal, and eukaryotic cells. All cells. Period," Darst says. "This particular study is about E. coli RNA polymerase but, as far as the active site and the catalytic mechanism are concerned, it's safe to say they're going to be exactly the same in other forms of life."
With a precise structural blueprint in hand, researchers can now begin to understand why mutations in this region of RNAP are so disruptive. For example, the new structure shows how altering the amino acids that hold the essential magnesium ions in place can completely destroy the enzyme's catalytic activity. The findings also provide a framework for studying how this universally conserved system has remained so stable over evolutionary time.
"Almost every residue around the active site is conserved across all organisms," Darst says. "With these structures, we will be better able to understand and interpret mutations in those residues, and figure out what they're actually doing and why they're so highly conserved."
