Activities of the RNAI and RNAII promoters of plasmid pBR322.

The synthesis rates of the replication control RNAs of plasmid pBR322, RNAI, an inhibitor of replication, and RNAII, the preprimer, have been determined by hybridizing in vivo pulse-labeled RNA to specific, single-stranded DNA probes for RNAI and RNAII. In Escherichia coli growing in glycerol minimal medium, RNAI transcripts were made at a rate of one molecule per 30 s per plasmid; RNAII was transcribed fivefold less, at a rate of one molecule per 3 min per plasmid. It is estimated that only 1 in 20 prepriming events leads to replication.