A ρ factor (rho factor) is a protein found in prokaryotes, especially E. coli, involved in the termination of transcription. This is done by dissociating the ternary transcription complex at the termination of a gene.

Steps involved

1. An 80 to 100-residue ρ-dependent terminator composed of a sequence rich in cytosine but poor in guanine before the actual termination site is recognized by the ρ factor on the mRNA segment being synthesized by RNA polymerase.
2. The ρ factor moves along the mRNA segment towards the polymerization site.
3. The fact that RNA polymerase pauses at the termination site on the DNA transcribed allows the ρ factor to catch up with RNA polymerase.
4. The helicase activity of the ρ factor separates the DNA-RNA hybrid at the polymerization site and transcription is stopped releasing the mRNA, the ρ factor and the RNA polymerase.

Mutations in a ρ factor may cause the RNA polymerase to read through from one operon to the next, creating longer mRNA molecules.