REPLICATION AND MAINTENANCE OF LAMBDA-PLASMIDS DEVOID OF THE CRO REPRESSOR AUTOREGULATORY LOOP IN ESCHERICHIA-COLI

Plasmids derived from bacteriophage lambda are known as lambda plasmid s. These plasmids contain the ori lambda. region and lambda replicatio n genes O and P. Typical lambda plasmids also contain the cro gene, th e product of which is a repressor of the p(R) promoter when present at relatively high concentrations. These genes stably maintain the plasm id in Escherichia coli at copy numbers of 20 to 50 per cell. According to a generally accepted model, stable maintenance of lambda plasmids is possible due to the Cro repressor autoregulatory loop (the cro gene is under control of p(R)). Here we demonstrate that lambda plasmids d evoid of the Cro autoregulatory loop can also be stably maintained in E. coli strains. We present data for two such plasmids: pTC lambda 1 i n which the p(R)-cro region has been replaced by the p(tetA) promoter and the tetR gene (coding for the TetR repressor), and a standard lamb da plasmid with inactivated cro gene (lambda cro-null plasmid). Thus, the presence of the Cro repressor autoregulatory loop does not appear to be essential to the maintenance of lambda plasmids in vivo. (C) 199 8 Academic Press.