Replication and Maintenance of λ Plasmids Devoid of the Cro Repressor Autoregulatory Loop inEscherichia coli

Abstract Plasmids derived from bacteriophage λ are known as λ plasmids. These plasmids contain the ori λ region and λ replication genes O and P . Typical λ plasmids also contain the cro gene, the product of which is a repressor of the p R promoter when present at relatively high concentrations. These genes stably maintain the plasmid in Escherichia coli at copy numbers of 20 to 50 per cell. According to a generally accepted model, stable maintenance of λ plasmids is possible due to the Cro repressor autoregulatory loop (the cro gene is under control of p R ). Here we demonstrate that λ plasmids devoid of the Cro autoregulatory loop can also be stably maintained in E. coli strains. We present data for two such plasmids: pTCλ1 in which the p R – cro region has been replaced by the p tetA promoter and the tetR gene (coding for the TetR repressor), and a standard λ plasmid with inactivated cro gene (λ cro -null plasmid). Thus, the presence of the Cro repressor autoregulatory loop does not appear to be essential to the maintenance of λ plasmids in vivo.