Integrative suppression of a dnaA mutation in Salmonella typhimurium

Integrative suppression of a dnaA mutation in Salmonella typhimurium may result from the integration of F'lac or F'his into the chromosome in the left hand side of the chromosomal map. The suppressed revertants resulting from this integration do not contain DNA of the F' elements in the covalently closed circular (CCC)1 form but still contain the CCC DNA of the cryptic LT2 plasmid. Two suppressed revertants isolated from dnaA/F- strains were found in which the suppression of dnaA character was accompanied by the loss of CCC DNA from the cell lysates. From one of these revertants a segregant was isolated in which the return to the dnaA phenotype was accompanied by the reappearance of CCC DNA in the cell lysate. It is suggested that the cryptic plasmid may integrate into the chromosome of S. typhimurium and this integration may result in suppression of the dnaA mutation. Additional evidence suggesting that the cryptic plasmid controls its own initiation of replication independently of the function of the chromosomal dnaA gene is supplied by the results of the determination of incorporation of labelled thymidine into CCC DNA of the dnaA1 strain at the nonpermissive temperature.