A summary of " Ribsome reactivates transcription by physically pushing RNA polymerase out of transcription arrest"

 Bacteria lack a nuclear envelope, unlike eukaryotes due to which both transcription and translation is coupled. This has several roles, it helps in regulating gene expression, synchronizes the process and also mitigates backtracked elongation complexes. 

Backtracking is a phenomenon where the Elongation complex slides backward along the DNA template, resulting in the misalignment of the 3' end of the RNA transcript. 

A backtracked complex is pretty much stable and if not reactivated can cause genome instability, and collide with replication machinery, and transcriptional traffic. 

The elongation factors GreA and GreB can promote a transcript cleavage to resume transcription. However, it is not essential as coupling itself can play a role in mitigating a backtracked complex. Ribosomes prevent Elongation complexes from long backtracking as it can only translocate by 1 codon when both machineries are sufficiently close to each other.

However, some questions are still left unanswered. It is not known if the Ribosome reverses backtracking or if the backtracking causes some kind of roadblock. 

 A coupled transcription-translation system was developed to investigate the encounters of the EC and the ribosome. The translation initiation complex was formed on a 5' radiolabeled mRNA which was then coupled to transcription elongation complex with RNAP and template and non-template DNA that was complementary to the mRNA. This system was then immobilized with Streptavidin beads through a biotin tag on the 3' end of the non-template DNA, 

                                                                 Flint-Stevenson Jones Et.al, 2020

When the mRNA is allowed to transcribe the backtracked RNAP in the presence of RelE, it was observed that the ribosome could forward translocate the backtracked complex by roughly 10 base pairs and they remained coupled with ribosomes. From this it was concluded that backtracked RNAP do not inhibit translation rather the ribsome actively reverses backtracking. 

Based on the distance between GreB and RelE, it was seen that the distance between the RNAP active site and peptidyl transferase center of ribosome was observed to be around 25-26 base pairs. And there is no tight complex formed between RNAP and the Ribosome. 

The translation machinery is assisted by an elongation factor (EF-G). In case of translational backtracking, EF-G reverts it into the posttranslocated state. The translating ribsome itself can translocate the backtracked elongation complex by physically pushing it forward. This ensures that both the processes are synchronized and also avoid collisions between transcription and replication machinery. 

This study primarily shows that backtracked Elongation complexes do not inhibit translation on the nascent mRNA. From this result, a cooperative model is favored between the transcribing RNAP and the translating ribosomes although they don't form a stable functional complex between them, "but move along their respective templates independently". In the case of Ribosome lagging behind the EC and it backtracks, the ribosome catches up with it and activates the backtracked complex. Apart from this some other interesting inferences are made as well.