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

Temperature sensitivity of DNA polymerization and growth, resulting from mu tation of the tau and gamma subunits of Escherichia coli DNA polymerase III , are suppressed by Cs,Sx mutations of the initiator gene, dnaA. These muta tions simultaneously cause defective initiation at 20 degrees C. Efficient suppression, defined as restoration of normal growth rate at 39 degrees C t o essentially all the cells, depends on functional oriC. Increasing DnaA ac tivity in a strain capable of suppression, by introducing a copy of the wil d-type allele, increasing the suppressor gene dosage or introducing a seqA mutation, reversed the suppression. This suggests that the suppression mech anism depends on reduced activity of DnaACs,Sx. Models that assume that sup pression results from an initiation defect or from DnaACs,Sx interaction wi th polymerization proteins during nascent strand synthesis are proposed.