Figure 5.

DNA polymerase III actively shapes the outcome of TLS reaction in vivo. This notion is illustrated by the Pol II-mediated TLS pathway at the NarI site in the presence of a GAAF adduct (shown in red). In vitro, Pol II generates both −1 and −2 frameshift TLS products, whereas in vivo only −2 frameshift events are made. Minus 1 frameshift events result from frequent misinsertion by Pol II of G opposite GAAF, slippage, and subsequent elongation. It turns out that these −1 frameshift events are aborted by Pol III proofreading. The addition of Pol III to the Pol II-mediated bypass reaction in vitro allows the in vivo situation to be mimicked.