Plasmids carrying the intact Bacillus subtilis dnaA-like gene and two reciprocal hybrids between the B. subtilis and Escherichia coli dnaA genes were constructed. None of the plasmids could transform wild-type E. coli cells unless the cells contained surplus E. coli DnaA protein (DnaAEc). A dnaA (Ts) strain integratively suppressed by the plasmid R1 origin could be transformed by plasmids carrying either the B. subtilis gene (dnaABs) or a hybrid gene containing the amino terminus of the E. coli gene and the carboxyl terminus of the B. subtilis gene (dnaAEc/Bs). In cells with surplus E. coli DnaA protein, expression of the E. coli dnaA gene was derepressed by the B. subtilis DnaA protein and by the hybrid DnaAEc/Bs protein, whereas it was strongly repressed by the reciprocal hybrid protein DnaABs/Ec. The plasmids carrying the different dnaA genes probably all interfere with initiation of chromosome replication in E. coli by decreasing the E. coli DnaA protein concentration to a limiting level. The DnaABs and the DnaAEc/Bs proteins effect this decrease possibly by forming inactive oligomeric proteins, while the DnaABs/Ec protein may decrease dnaAEc gene expression.