Mechanism of DNA polymerase II-mediated frameshift mutagenesis

Escherichia coil possesses three SOS-inducible DNA polymerases (Pol II, IV, and V) that were recently found to participate in translesion synthesis an d mutagenesis. Involvement of these polymerases appears to depend on the na ture of the lesion and its local sequence context, as illustrated by the by pass of a single N-2-acetylaminofluorene adduct within the Narl mutation ho t spot. Indeed, error-free bypass requires Pol V (umuDC), whereas mutagenic (-2 frameshift) bypass depends on Pol II (polB), In this paper, we show th at purified DNA Pol II is able in vitro to generate the -2 frameshift bypas s product observed in vivo at the Narl sites, Although the Delta polB strai n is completely defective in this mutation pathway, introduction of the pol B gene on a low copy number plasmid restores the -2 frameshift pathway. In fact, modification of the relative copy number of polB versus umuDC genes r esults in a corresponding modification in the use of the frameshift versus error-free translesion pathways, suggesting a direct competition between Po l II and V for the bypass of the same lesion, Whether such a polymerase com petition model for translesion synthesis will prove to be generally applica ble remains to be confirmed.