FIG 1.

ppGpp inhibits DNA replication initiation in E. coli. (A) Schematic of replication initiation in E. coli and the prevailing model for inhibition by ppGpp. There are three major steps to initiation. (i) DnaA (green circles) assembles onto the double-stranded DnaA-binding region (green). (ii) The DnaA filament spreads onto the melted, single-stranded DNA unwinding element (DUE [cyan]). (iii) DnaA recruits components of the replisome (orange) to initiate DNA replication. (B) Representative flow cytometry profiles of wild-type E. coli grown in LB. Cells were grown to an OD₆₀₀ of ∼0.2, treated as follows, and then fixed for analysis: pretreatment (left panel), addition of 1 mg/ml serine hydroxamate for 90 min (middle panel), induction of RelA′ with 100 μM IPTG for 90 min (right panel). (Bottom) Immunoblot for DnaA in cultures corresponding to flow cytometry profiles in panel A. The control is a nonspecific band seen with the DnaA antibody.