During DNA transcription, RNA polymerases often adopt inactive backtracked 
states. Recovery from backtracks can occur by 1D diffusion or cleavage of 
backtracked RNA, but how polymerases make this choice is unknown. Here, we use 
single-molecule optical tweezers experiments and stochastic theory to show that 
the choice of a backtrack recovery mechanism is determined by a kinetic 
competition between 1D diffusion and RNA cleavage. Notably, RNA polymerase I 
(Pol I) and Pol II recover from shallow backtracks by 1D diffusion, use RNA 
cleavage to recover from intermediary depths, and are unable to recover from 
extensive backtracks. Furthermore, Pol I and Pol II use distinct mechanisms to 
avoid nonrecoverable backtracking. Pol I is protected by its subunit A12.2, 
which decreases the rate of 1D diffusion and enables transcript cleavage up to 
20 nt. In contrast, Pol II is fully protected through association with the 
cleavage stimulatory factor TFIIS, which enables rapid recovery from any depth 
by RNA cleavage. Taken together, we identify distinct backtrack recovery 
strategies of Pol I and Pol II, shedding light on the evolution of cellular 
functions of these key enzymes.