Fig. 7. The three-route model of ρ-dependent termination.

Three kinetically different routes of transcription termination are operating with a single terminator in our model for ρ-dependent termination, whereas all previous models assume a single path. (1) Catch-up recycling termination. The fastest termination route is taken by catch-up ρ, which binds nascent RNA at a rut site, translocates down on RNA, and catches up with RNAP that is pausing at a termination site. Only RNA, not RNAP, is released, and post-terminational RNAP recycles on DNA for 1D reinitiation (like in most intrinsic termination^15–17, which is ρ-independent but RNA hairpin-dependent). (2) Catch-up decomposing termination. If the first, fastest termination fails, the second termination route can be followed by catch-up ρ. Transcription complex decomposes in a step as both RNA and RNAP depart DNA. This second-route termination by catch-up ρ for decomposing always occurs much later than the first-route termination for recycling. (3) Stand-by decomposing termination. The slowest termination route is usually pursued by stand-by ρ, which pre-binds RNAP, moves along DNA with RNAP, and stands by for binding a rut site emerging out of RNAP. This termination results in one-step disassembly of the transcription complex. Interestingly, stand-by ρ presumably binds RNAP earlier than catch-up ρ but tends to achieve termination not so much earlier. While the catch-up decomposing termination route (2) is the most frequent in all terminators, relative frequencies of the other routes (1) and (3) depend on terminators.