Impact of glucose uptake rate on recombinant protein production in Escherichia coli

Escherichia coli ( E. coli ) is an attractive host for production of recombinant proteins, since it generally provides a rapid and economical means to achieve high product quantities. In this thesis, the impact of the glucose uptake rate on the production of recombinant proteins was studied, aiming at improving and optimising production of recombinant proteins in E. coli. E. coli can be cultivated to high cell densities in bioreactors by applying the fed-batch technique, which offers a means to control the glucose uptake rate. One objective of this study was to find a method for control of the glucose uptake rate in small-scale cultivation, such as microtitre plates and shake flasks. Strains with mutations in the phosphotransferase system (PTS) where used for this purpose. The mutants had lower uptake rates of glucose, resulting in lower growth rates and lower accumulation of acetic acid in comparison to the wild type. By using the mutants in batch cultivations, the formation of acetic acid to levels detrimental to cell growth could be avoided, and ten times higher cell density was reached. Thus, the use of the mutant strains represent a novel, simple alternative to fed-batch cultures. The PTS mutants were applied for production of integral membrane proteins in order to investigate if the reduced glucose uptake rate of the mutants was beneficial for their production. The mutants were able to produce three out of five integral membrane proteins that were not possible to produce by the wild-type strain. The expression level of one selected membrane protein was increased when using the mutants and the expression level appeared to be a function of strain, glucose uptake rate and acetic acid accumulation. For production purposes, it is not uncommon that the recombinant proteins are secreted to the E. coli periplasm. However, one drawback with secretion is the undesired leakage of periplasmic products to the medium. The leakage of the product to the medium was studied as a function of the feed rate of glucose in fed-batch cultivations and they were found to correlate. It was also shown that the amount of outer membrane proteins was affected by the feed rate of glucose and by secretion of a recombinant product to the periplasm. The cell surface is another compartment where recombinant proteins can be expressed. Surface display of proteins is a potentially attractive production strategy since it offers a simple purification scheme and possibilities for on-cell p