Quantitative and Contextual Characterization of Plasmid Copy Number

Intracellular plasmid copy numbers directly affect the dosage of genetic constructs expressed from extrachromosomal replicons.  This is particularly relevant for complex, multigene systems that require precise gene expression levels to be functional (e.g., genetic circuits, metabolic pathways).  Furthermore, mathematical models of plasmid-based gene expression require more than a qualitative or semi-quantitative description of plasmid copy number to be accurate or meaningful.  We have developed a straightforward method for determining plasmid copy number using a simple mass balance-based model of gene expression with two measurable inputs, steady-state fluorescence and growth rate.  Using our approach, we have investigated various genetic, cellular and environmental context dependencies of plasmid copy number in commonly used laboratory strains of Escherichia coli.