Biosynthetic Burden and Plasmid Burden Limit Expression of Chromosomally Integrated Heterologous Genes (pdc, adhB) in Escherichia coli

Previous studies have shown an unexpectedly high nutrient requirement for efficient ethanol production by ethanologenic recombinants of Escherichia coli B such as LYOI which contain chromosomally integrated Zymomonas mobilis genes (pdc,adhB) encoding the ethanol pathway. The basis for this requirement has been identified as a media-dependent effect on the expression of the Z. mobilis genes rather than a nutritional limitation. Ethanol production was substantially increased without additional nutrients simply by increasing the level of pyruvate decarboxylase activity. This was accomplished by adding a multicopy plasmid containing pdc alone (but not adhB alone) to strain LYOI, and by adding multicopy plasmids which express pdc and adhB from strong promoters. New strong promoters were isolated from random fragments of Z. mobilis DNA and characterized but were not used to construct integrated biocatalysts. These promoters contained regions resembling recognition sites for 3 different E. coli sigma factors: Q^70, Q^38, and Q^28. The most effective plasmidbased promoters for fermentation were recognized by multiple sigma factors, expressed both pdc and adhB at high levels, and produced ethanol efficiently while allowing up to 80% reduction in complex nutrients as compared to LYO1. The ability to utilize multiple sigma factors may be advantageous to maintain the high levels of PDC and ADH needed for efficient ethanol production throughout batch fermentation. From this work, we propose that the activation of biosynthetic genes in nutrient-poor media creates a biosynthetic burden that reduces the expression of chromosomal pdc and adhB by competing for transcriptional and translational machinery. This reduced expression can be viewed as analogous to the effect of plasmids (plasmid burden) on the expression of native chromosomal genes.