On the possible role of cytosine deamination in delayed photoreversal mutagenesis targeted at thymine-cytosine dimers in E. coli.

While delayed photoreversal (PR) mutagenesis has been interpreted as a measure of misincorporation step in targeted mutagenesis, the specificity to produce glutamine tRNA suppressor mutations (C to T transitions) at sites in DNA where a thymine-cytosine dimer (T = C) may target mutation suggests a deamination model: deamination T = C to T = U and trans-U DNA replication after PR. We describe here two enquires that did not support the latter model: (a) Uracil DNA glycosylase activity as estimated from the restricted plating efficiency of phage T5 containing uracil-substituted DNA showed no variation that might allow an exceptional opportunity for mutation at U in DNA, and (b). The kinetics of delayed PR mutagenesis were unaltered if UV-irradiated cells were held in buffer suspension for 2 h at 41 degrees C (a procedure known to allow deamination T = C to T = U) and then assayed. Other results with cells containing both umuC and ung (uracil DNA glycosylase) defects showed the magnitude of T = C deamination sufficient to provide T = U at the critical site of mutation to an extent greater than the mutation frequencies produced by delayed PR mutagenesis, and considerations of the kinetics led to the suggestion that the deamination model could apply if there were an optimum period 30-130 min post-UV for efficient recovery of DNA replication after PR. The results underscored the feasibility of delayed PR mutagenesis by deamination and trans-U replication, but a selection between the two models could not be determined.