3.4 Host Strains
The most commonly used host strains have been derived from E. coli K-12, although other bacteria, yeast cells, plant cells and mammalian cells are used according to the needs of the experiment. For example, if you are producing a human protein that has specific carbohydrate residues needed for activity, it may be essential to produce that protein in mammalian cells. Also, eukaroytic genes usually have introns and exons that cannot be directly transcribed into useful mRNA by prokaryotes, such as E. coli (see molecular biology and cDNA libraries sections). Sometimes a bacterial host may be used for initial experiments and gene isolation, and then the vector is moved to a mammalian host for expression.
Our examples use E. coli K-12 strains that have the following properties:
E. coli is a Gram-negative short rod. The genome consists of a large, compactly folded circle containing 4 x 106 base pairs. E. coli proteins are made on ribosomes in the cytoplasm and either remain there or are extruded into the periplasmic space, or even excreted into the medium by special mechanisms. Recombinant proteins often accumulate as insoluble inclusions in the cytoplasm.
Figure 19. E. coli is a commonly used host cell.