Genetic inactivation of topoisomerase I suppresses a defect in initiation of chromosome replication in Escherichia coli.

A strain of Escherichia coli K12 harboring simultaneously the temperature-sensitive dnaA46 mutation and a deletion of the trp-topA-cysB region plates with the same full efficiency at 30 degrees C and 42 degrees C. We have analyzed the possible involvement of the gene coding for topoisomerase I, topA, in this suppression phenomenon. The Ts phenotype was retrieved upon introduction of a plasmid-borne DNA fragment including an active topA gene into this strain, but not upon introduction of the same fragment harboring a topA::Tn1000 insertion. Replication seems to remain DnaA-dependent in the delta (topA) strain, however, since we have been unable to introduce a dnaA::Tn10 allele. We propose either that the dnaA46 gene product is overproduced and compensates for its thermal inactivation, or that initiation at oriC demands less DnaA protein in the absence of topoisomerase I.