The RNA primer promoter, as defined in vitro, is essential for pMB1 plasmid replication in vivo

The fusion of the β-galactosidase structural gene to a DNA fragment that contains the promoter of the primer for initiation of pMB1 replication in vitro has allowed the isolation of a mutation in this promoter. By recombination in vivo, this mutation was introduced into a plasmid that contains all the information essential for plasmid replication. Since the resulting mutant plasmid is not able to replicate, it is concluded that transcription starting from the promoter of the in vitro primer precursor is essential for pMB1 replication.