Roles of E-coli DNA polymerases IV and V in lesion-targeted and untargetedSOS mutagenesis

The expression of the Escherichia coli DNA polymerases pol V (UmuD'(2) C co mplex)(1,2) and pol IV (DinB)(3) increases in response to DNA damage(4). Th e induction of pol V is accompanied by a substantial increase in mutations targeted at DNA template lesions in a process called SOS-induced error-pron e repair(4). Here we show that the common DNA template lesions, TT (6-4) ph otoproducts, TT cis-syn photodimers and abasic sites, are efficiently bypas sed within 30 seconds by pol V in the presence of activated RecA protein (R ecA*), single-stranded binding protein (SSB) and pol III's processivity bet a,gamma-complex. There is no detectable bypass by either pol IV or pol III on this time scale. A mutagenic 'signature' for pol V is its incorporation of guanine opposite the 3'-thymine of a TT (6-4) photoproduct, in agreement with mutational spectra. In contrast, pol III and pol IV incorporate adeni ne almost exclusively. When copying undamaged DNA, pol V exhibits low fidel ity with error rates of around 10(-3) to 10(-4), with pol IV being 5- to 10 -fold more accurate. The effects of RecA protein on pol V, and beta,gamma-c omplex on pol IV, cause a 15,000- and 3,000-fold increase in DNA synthesis efficiency, respectively. However, both polymerases exhibit low processivit y, adding 6 to 8 nucleotides before dissociating. Lesion bypass by pol V do es not require beta,gamma-complex in the presence of non-hydrolysable ATP g amma S, indicating that an intact RecA filament may be required for transle sion synthesis.