Molecule of the Month: Expressome

In our cells, transcription and translation occur in two different places: DNA is transcribed into messenger RNA in the nucleus, and messenger RNA is translated into proteins in the cytoplasm. Bacterial cells, on the other hand, do everything in one place, creating the possibility that the two processes may interfere with one another. In reality, however, bacteria take advantage of this. Ribosomes immediately start work on messenger RNA as it is transcribed, and end up tugging on the new mRNA enough that they promote continued formation of the mRNA.

In general, however, bacteria don't simply allow ribosomes to crash into RNA polymerase. Instead, they use transcription elongation factors to link the RNA polymerase to the ribosome, holding it close, but not too close. These transcription elongation factors, such as NusG and NusA, are flexible and allow some space between the polymerase and ribosome, leaving room for the many small ribosomal motions that are needed as amino acids are added to the new protein..

RNA polymerase, a ribosome, and all the associated molecules together form an "expressome," or "transcription-translation complex" (TTC). As seen in PDB entry 6x9q, NusG and NusA bridge between RNA polymerase and the small subunit of the ribosome, placing the emerging mRNA strand in the perfect place for entry into the ribosome.

As you can imagine, these large complexes are difficult to study, so researchers are taking several approaches to visualize them. An early study, available in PDB entry 5my1 (not shown), paused RNA polymerase and allowed the ribosome to crash into it. Recently, other researchers used a short piece of RNA to tie the polymerase and the ribosome together, as in PDB entry 6ztm (shown on the left) and PDB entry 6vu3 (not shown), finding that this creates complexes similar to the earlier collided structure. By comparison, we can see that the NusG/NusA transcription elongation factors (shown on the right) hold the polymerase and ribosome a bit further apart, and in a quite different relative orientation.