EXPERIMENTAL FIGURE 6-28 Some eukaryotic proteins can be produced in E. coli cells from plasmid vectors containing the lac promoter. (a) This plasmid expression vector contains a fragment of the E. coli chromosome containing the lac promoter and the neighboring lacZ gene. In the presence of the lactose analog IPTG, RNA polymerase normally transcribes the lacZ gene, producing lacZ mRNA, which is translated into the encoded protein, β-galactosidase. (b) The lacZ gene can be cut out of the expression vector with restriction enzymes and replaced by a cloned cDNA, in this case, one encoding granulocyte colony-stimulating factor (G-CSF). When the resulting plasmid is inserted into E. coli cells, addition of IPTG and subsequent transcription from the lac promoter produce G-CSF mRNA, which is translated into G-CSF protein.