Figure 2.

Structure of replication intermediates and involvement of SOS factors. (A) AAF-induced frameshift mutagenesis. When AAF adducts are situated in repetitive sequence contexts associated with both −1 and −2 frameshifts, the TLS intermediates can assume two forms: a nonslipped intermediate or a slipped intermediate. Elongation from the nonslipped intermediate, i.e., error-free elongation, proceeds from a lesion terminus at which the terminal nucleotide of the nascent strand is situated across from the AAF lesion (circled). Elongation from the lesion terminus appears to be stimulated by UmuDC. In contrast, frameshift mutations result from the elongation of the postlesion terminus of the slipped intermediate, at which the terminal nucleotide of the nascent strand is correctly paired with residues situated 5′ to the adduct on the template strand (shown in the square). G* is the AAF adducted guanine residue. (B) UV-induced base substitution mutagenesis. Both error-free and mutagenic TLS across a photoproduct involve elongation from a lesion terminus because, in the intermediate, the terminal nucleotide in the nascent strand is situated across from the lesion (circled). Both elongation events are strongly stimulated by the presence of UmuDC.