For the first time, scientists have captured the bacterial enzyme RNA polymerase at the precise moment before it adds a new RNA nucleotide.
Researchers capture the 'moment before creation' in RNA synthesis
A landmark study published in Molecular Cell has used cryo-electron microscopy (cryo-EM) to resolve a long-standing question about how the enzyme RNA polymerase (RNAP) catalyzes the addition of new RNA building blocks.
Methodology and Key Findings
To capture this fleeting pre-catalytic state, the research team supplied E. coli RNAP with only three of the four required RNA building blocks. This caused the enzyme to repeatedly initiate and then stall the production of short RNA fragments—a process called abortive transcription.
The team flash-froze these samples in liquid ethane and imaged approximately 2 million individual particles using cryo-EM. This allowed them to construct near-atomic resolution models that resolved individual ions and water molecules.
The resulting structures reveal a near-perfect spatial alignment of the growing RNA chain with the incoming nucleotide. Critically, the models show a continuous chain of water molecules extending from the active site to the surrounding solution.
"This water network provides a path for proton removal, supporting a water-mediated mechanism of catalysis rather than direct involvement by the enzyme's own amino acids."
The reaction is known to require two magnesium ions, with a flexible region of the enzyme—called the trigger loop—folding to position the catalytic components.
Conservation and Implications
The study also observed the same water-based architecture in related enzymes from yeast, suggesting the mechanism is conserved across bacteria, archaea, and eukaryotes.
The findings establish a structural blueprint for RNA synthesis catalysis, as the active site architecture is nearly identical in all three domains of life.
These structures provide a framework for interpreting mutations in conserved residues around the active site, potentially explaining why specific mutations disrupt catalytic activity. Future research is expected to examine how the enzyme adapts to different DNA-RNA base pair combinations while maintaining transcriptional accuracy.
Statements from Researchers
Andreas Mueller, a research associate at the Laboratory of Molecular Biophysics, stated that this is the first time structures have been solved during an ongoing reaction, providing the closest structural snapshot to date of the transition state.
Seth Darst, head of the laboratory, noted that while this study specifically examined E. coli RNA polymerase, the active site and catalytic mechanism are expected to be identical in other forms of life.