AUTOREGULATION OF THE ESCHERICHIA-COLI REPLICATION INITIATOR PROTEIN,DNAA, IS INDIRECT

The expression of dnaA is autoregulated, in that transcription of the gene increases when DnaA is inactivated (and initiation of replication prevented) and decreases when DnaA is supplied in excess. However, th e inactivation of DnaA does not necessarily lead to increased DnaA pro duction, as dnaA(Ts; temperature sensitive) strains which are integrat ively suppressed by derivatives of the plasmid R1 do not show temperat ure-induced derepression. Several possible explanations for this unant icipated behaviour were considered and ruled out. We suggest here that the completion of a critical step in initiation may prevent dnaA dere pression: although DnaA would be required to complete this step at ori C, DnaA(Ts) would be sufficient at the R1 origin. Autoregulation of dn aA has been attributed to the binding of DnaA at a consensus binding s ite in the dnaA promoter region. We show here, using reporter systems, that this DnaA-binding site is not required for the autoregulatory re sponse. We find, further, that replacement of the chromosomal dnaA gen e with one containing a mutated binding site causes no demonstrable ph enotypic change: cells with the mutant gene show no disadvantage in co mpetition with dnaA(+) cells.