Expression and purification of rhTyrRS. A: Fed-batch growth curve of BL21 cells expressing rhTyrRS. A 5-L fermentor was inoculated with a 150-mL seed culture at time 0 and samples were drawn at regular intervals to monitor the optical density at 600 nm. rhTyrRS expression was induced with IPTG at approximately OD600 = 30. B: SDS-PAGE analysis of rhTyrRS expressed by fed-batch fermentation. Lane 1, standard protein marker (20–250 kD); Lane 2, uninduced BL21 cells; Lanes 3–8, BL21 cells induced with 0.5 mM IPTG for 1, 2, 3, 4, 5, and 6 h.