Mutation spectra of TA, the major photoproduct of thymidylyl-(3′-5′)-deoxyadenosine, in Escherichia coli under SOS conditions

The biological activity of TA*, the major photoproduct of thymidylyl-(3′,5′)-deoxyadenosine, has remained speculative since it was identified a decade ago. To determine the mutagenicity of TA* in Escherichia coli, we constructed the replicative form of an M13mp18-derived phage containing TA* in the (-)-strand by polymerase-catalyzed elongation of a TA*-containing 49mer opposite a uracil-containing (+)-strand of the phage. The in vitro synthesis mixture was transfected into an ung⁺, phr^- E.coli host and the progeny were screened with a hybridization probe unique for the (-)-strand. TA* was found to block DNA replication substantially in the absence of SOS, but under SOS, TA* was bypassed more efficiently and was highly mutagenic. Among 56 analyzed (-)-strand progeny from two transfections, 46 (82%) were mutants, including six (11%) tandem mutants. The most abundant mutation was a 3′ A→T substitution (31/46, 56%), The possible biological consequences of TA* formation in the highly conserved TATA box consensus sequence on gene expression are discussed in light of the mutagenicity of TA*.