DnaA-mediated regulation of phage lambda-derived replicons in the absence of pR and Cro function

Bacteriophage lambda-derived replicons can replicate in Escherichia coli cells as plasmids. In the control of replication of these plasmids, an important role was ascribed to the lambda Cro repressor autoregulatory loop. However, the oR/pR-cro-tR-cII' region could be replaced by the ptetA promoter under the control of the TetR repressor, producing plasmid pTClambda. Here, we demonstrate that stable maintenance of pTClambda depends on the host DnaA function because deletion of one of DnaA-binding sequences present in pTClambda resulted in a decrease in the plasmid (pTClambda) copy number and poor maintenance of pTClambda in E. coli. Moreover, in contrast to the replication of the wild-type lambda plasmid, previously found to be positively regulated by DnaA (acting on a relaxed DnaA box situated immediately downstream of the pR promoter), the replication of pTC plasmids (devoid of pR) was found to be negatively regulated by DnaA. Contrary to wild-type lambda plasmids, in cells harboring lambda cro[temperature-sensitive (ts)] or pTClambda (but not pTClambda) plasmid, the lambda replication complex was heat shock resistant; this complex, however, disassembled after inactivation of DnaA function. This disassembly was blocked by DNA gyrase inhibitors. According to our model outlined previously, we propose that the heat shock resistance of the replication complex of lambdacro- plasmids depends on the interaction of the DNA-bound DnaA protein with the DNA-bound lambda replication complex. The replication complex-DnaA-lambda DNA structure may be directly related to the role of DnaA as the Cro-replacing negative regulator of lambdacro- plasmid replication.