In bacteria two distinct types of termination events have been identified-those that depend on the action of a protein termination factor, called (rho), and those that are factor - independent.

Factor-dependent termination - Factor-dependent termination sites are less frequent than factor-independent termination sites, and the mechanism of factor-dependent termination is complex. The protein, a hexamer composed of identical subunits, has been characterized as an RNA--DNA helicase (see here for more about helicases) and contains a nucleoside triphosphatase activity that is activated by binding to polynucleotides. Apparently acts by binding to the nascent transcript at a specific site near the 3' end, when RNA polymerase has paused (Figure 26.16). Then moves along the transcript toward the 3' end, with the helicase activity unwinding the 3' end of the transcript from the template (and/or the RNA polymerase molecule), thus causing it to be released.

Involvement of NusA protein - It is not clear what causes RNA polymerase to pause at -dependent termination sites. However, the action of another protein, NusA, is somehow involved. The NusA protein evidently associates with RNA polymerase, and there is reason to believe that it binds at some point in transcription after the factor has dissociated, because the two purified proteins compete with each other for binding to core RNA polymerase.

Attentuation - Further insight into termination mechanisms has come from an extensively studied regulatory mechanism called attenuation (Figure 26.33, Figure 26.36). Attenuation controls the rate of transcription of certain operons by terminating the synthesis of a nascent transcript before RNA polymerase has reached the structural genes.