The formation of pyridine-pyrimidine and pyrimidine-pyrimidine base pairs after in vitro DNA replication with the large fragment of Escherichia coli DNA polymerase I indicates that Watson-Crick-like base pairing between pyrimidine bases might occur in the enzyme due to the presence of the rare tautomers of deoxycytidylate and thymidylate in the template strand. The implications for mispair formation in DNA, such as the difference between the structures of the mispairs during and after replication, are discussed and the possible action of mutagenic DNA protonating and deprotonating agents in vivo is considered.
 
 

Figure: Replication experiment with 3-deazacytosine as template base N

 
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