The effects of the environment on the stability and expression of genetically engineered plasmids
The lac promoter is widely used in plasmid expression systems, even though it is prone to catabolite repression. As a consequence glycerol is often used as an alternative carbon source. Three plasmids containing various sizes of the staphylococcal protein A (SPA) gene, which are under the control of the lac promoter were investigated in continuous culture, to evaluate the effects of nutrient limitations on their stability and expression. The fears of catabolite repression were dispelled as a low expression plasmid (pPA16) produced a greater amount of truncated SPA under glucose limiting conditions (11 ug mg-1 cell protein) when compared to that using glycerol (8 ug mg-1 cell protein). Segregational instability was also observed under glycerol limiting conditions at all the dilution rates investigated. Whereas pPA16 was relatively stable under glucose limiting conditions, with SPA production being continuous. Experiments using excess glycerol with limited ammonium increased the stability of pPA16, (when compared to limited glycerol) with expression of SPA being continuous but reduced (6 ug mg-1 cell protein). With excess glucose and limited ammonium the copy numbers remained high but expression of SPA paralled that produced under glucose limiting conditions. This might indicate that the higher levels of glucose are reducing expression (catabolite repression) or that the low level of ammonium is affecting protein production. A high expression plasmid (pPA31) produced nearly 100 ug full length SPA mg-1 cell protein, while another high expression plasmid (pPA34) producing truncated SPA proved to be very unstable. An ELISA was developed to detect the SPA produced by these experiments, which could be adapted for western blotting or immunogold probing using electron microscopy. SPA was localised in electron lucent areas present in the periplasmic space of the E. coli host harbouring pPA16. While in the same host containing pPA31, SPA was localised not only in electron lucent areas but also around the whole of the outer-membrane.