Suppression of a DnaX temperature-sensitive polymerization defect by mutation in the initiation gene, dnaA, requires functional oriC

Abstract

Temperature sensitivity of DNA polymerization and growth, resulting from mutation of the τ and γ subunits of Escherichia coli DNA polymerase III, are suppressed by Cs,Sx mutations of the initiator gene, dnaA. These mutations simultaneously cause defective initiation at 20°C. Efficient suppression, defined as restoration of normal growth rate at 39°C to essentially all the cells, depends on functional oriC. Increasing DnaA activity in a strain capable of suppression, by introducing a copy of the wild-type allele, increasing the suppressor gene dosage or introducing a seqA mutation, reversed the suppression. This suggests that the suppression mechanism depends on reduced activity of DnaACs,Sx. Models that assume that suppression results from an initiation defect or from DnaACs,Sx interaction with polymerization proteins during nascent strand synthesis are proposed.