Transcription is one of the most important steps in gene expression. During the elongation phase, the transcription complex moves along double-stranded DNA creating a transcription bubble by local unwinding of the helix (Transcription). As RNA is synthesized it forms a transient DNA:RNA helix at the active site of the enzyme (How RNA Polymerase Works: The Chemical Reaction). We now know what this transcription bubble really looks like, thanks to the work of 2006 Nobel Laureate Roger Kornberg
The figure on the left is taken from a review in Science magazine written by Aaron Klug (A Marvellous Machine for Making Messages). It shows the structure of the RNA polymerase II complex (Eukaryotic RNA polymerases) associated with a DNA:RNA hybrid that Kornberg's lab synthesized. They solved the structure of the co-crystal.
The solid blue and green lines represent fragments of DNA. As you can see from the diagram it is in the form of a double helix at the front end of RNA polymerase where it enters the groove on the leading edge. (The transcription complex is moving from left to right.) The DNA is gripped by the "jaw" region near the opening of the grove.
As the double-stranded region reaches the active site (identified by the purple Mg2+ ion), it unwinds to a single-stranded form creating a bubble. The bubble isn't actually seen in the crystal structure but its location can be inferred (dotted green and blue lines).
It looks like the unwinding is promoted when the DNA runs into the "wall" and is forced to make a sharp upward turn before exiting near the "clamp" where the two strands of DNA come back together to form a helix.
The blue strand of the transcription bubble is the template strand and part of it is associated with a short strand of RNA (red) behind the active site. There's a large funnel at the bottom of the enzyme that serves as a pathway from the outside to the site of polymerization. This is where nucleoside triphosphates (NTPs) enter and leave the active site. It also appears to be the site where the 3′ end of the RNA is extruded when the enzyme backs up for proofreading (backtracking).
The "bridge" part of the enzyme is required for the translocation step. This is the step following addition of a ribonucleotide when the enzyme has to shift by one nucleotide (base pair) to the right. The new 3′ end of RNA has to be re-positioned at the active site during this shift. At the same time, one base pair of DNA is unwound by the "fork" region of the enzyme and one base pair is reformed at the back end of the bubble by the "zipper" region.
The "bridge" acts like a flexible ratchet allowing a shift of one base pair while maintaining a grip on the growing end of the RNA molecule. This movement is steered by the "rudder."
Most of these terms ("bridge," "rudder" etc.) refer to short α helices or loops within RNA polymerase and almost all of them are part of the conserved β and β′ subunits. The same features are seen in the bacterial enzymes although the resolution of the bacterial enzyme structures is not good enough to decipher the translocation step. This is one of the achievements of the Kornberg group in the two famous papers (Gnatt et all, 2001; Cramer et al., 2001).
Gnatt, A.L., Cramer, P. , Fu, J., Bushnell, D.A., and Kornberg, R.D. (2001) Structural Basis of Transcription: An RNA Polymerase II Elongation Complex at 3.3 Å Resolution. Science 292:1876 - 1882.
Cramer, P. , Bushnell, S.A. and Kornberg, D.A. (2001) Structural Basis of Transcription: RNA Polymerase II at 2.8 Ångstrom Resolution. Science 292:1863 - 1876.