The conversion of single-stranded circular DNA of bacteriophage M13 or φX174 to the double-stranded form by cell-free extracts of
Escherichia coli requires Mg
2+ as a cofactor. In this report, it was examined whether any other divalent cation could substitute for Mg
2+ as a cofactor at the replication of these DNAs. The results showed that Mn
2+, Co
2+, Sr
2+, and Ba
2+ could replace Mg
2+ The replacement, however, induced some abnormal synthesis of DNA. The DNA synthesized with Mn
2+ or Sr
2+ contained several interruptions in the synthesized strand. The synthesis of DNA seems to be initiated at several points on the identical template. The DNA synthesized with Mn
2+, Co
2+, or Sr
2+ had plaque-forming ability, and the product synthesized with Mn
2+, or Co
2+ but not with Sr
2+, showed high frequencies of mutation. Although the synthesis of φX174 DNA was not inhibited by rifampicin when Mg
2+ was used as a cofactor, it was inhibited when Mn
2+ or Sr
2+ was used instead of Mg
2+.
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