Abstract
A system is described that enables the cloning of genes specifying detrimental proteins in Escherichia coli. The system is based on pUC plasmids and was developed for the expression of the Bacillus subtilis csaA gene, which is lethal when expressed at high levels. Suppressor strains that tolerate the presence of plasmids for high-level expression of csaA were isolated, which contained small cryptic deletion variants of the parental plasmid in high copy numbers. The cryptic plasmids consisted mainly of the pUC replication functions and lacked the csaA region and selectable markers. The co-resident, incompatible, cryptic plasmids enabled the maintenance of the csaA plasmids by reducing their copy number 20-fold, which resulted in a concomitant 3- to 7-fold reduction in the expression of plasmid-encoded genes. Strains carrying these cryptic endogenous plasmids proved to be useful for the construction of pUC-based recombinant plasmids carrying other genes, such as the skc gene of Streptococcus equisimilis, which cannot be cloned in high copy numbers in E. coli. Several strategies to reduce production levels of heterologous proteins specified by plasmids are compared.
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