Fig 3.

DsRed-Express2 and its derivatives are noncytotoxic when expressed at high levels in bacteria. (A) Representative colony size assay showing the bacterial cytotoxicities of several red and orange FPs. DH10B cells harboring the pREP4 repressor plasmid were transformed with pQE-60NA encoding either DsRed-Express2, E2-Red/Green, E2-Orange, mOrange2, KO, or KO* (see Figure 2 legend). In this assay, cells expressing DsRed-Express2, E2-Red/Green, and E2-Orange produced large colonies, indicating low cytotoxicity. Cells expressing mOrange2 and KO* produced very small or pinprick colonies, respectively, indicating cytotoxicity. Cells expressing KO produced large colonies, but these colonies were nearly colorless due to poor expression. (B) Quantitation of FP expression under derepressing conditions. Cells were grown to an OD₆₀₀ of ~0.6 and then treated with 1 mM IPTG for 4 h. Whole-cell lysates were separated using SDS-PAGE followed by staining with Coomassie Blue. Control cells were transformed with the empty pQE-60NA vector. The image shows that KO epxression was very low, while KO* expression was comparable to that of other FPs. (Reproduced with permission from ref. 18.)