Replication and Maintenance of &lgr; Plasmids Devoid of the Cro Repressor Autoregulatory Loop in Escherichia coli

Author： Herman-Antosiewicz A. Śrutkowska S. Taylor K. We˛grzyn G.

ISSN： 0147-619X

Source： Plasmid, Vol.40, Iss.2, 1998-09, pp. : 113-125

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Abstract

Plasmids derived from bacteriophage &lgr; are known as &lgr; plasmids. These plasmids contain the ori&lgr; region and &lgr; replication genes O and P. Typical &lgr; 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 &lgr; plasmids is possible due to the Cro repressor autoregulatory loop (the cro gene is under control of p_R). Here we demonstrate that &lgr; plasmids devoid of the Cro autoregulatory loop can also be stably maintained in E. coli strains. We present data for two such plasmids: pTC&lgr;1 in which the p_R–cro region has been replaced by the p_tetA promoter and the tetR gene (coding for the TetR repressor), and a standard &lgr; plasmid with inactivated cro gene (&lgr;cro-null plasmid). Thus, the presence of the Cro repressor autoregulatory loop does not appear to be essential to the maintenance of &lgr; plasmids in vivo.