Probable component of primosome involved in the initiation of DNA replication. It is essential for both replication initiation and membrane attachment of the origin region of the chromosome and plasmid pUB110. The two mutants dna-1 (dnaBI) and dnaB-19 (dnaBII) show different characteristics for replication and membrane binding of plasmid pUB110. DnaBI is essential for both chromosome and pUB110 replication, whereas dnaBII is necessary only for chromosome replication.