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To more directly assess the control of protein expression in this cell line, we performed a Western blot. Lysates from uninduced cells and cells induced with increasing amounts of ponA were fractionated by SDS-PAGE, blotted, and probed with a-luciferase polyclonal antisera (Figure 5C ). No significant detectable luciferase is expressed in the uninduced extract, whereas a linear increase in signal is observed from 300 nM to 5 M ponA. In a time-course experiment, a five-fold induction was achieved only 1 hour after adding 10 mM ponA, and a linear increase was observed for up to 20 hours, at which time an induction ratio of 1,030-fold was observed (Figure 5D ). We also observed these induction kinetics in transient assays (data not shown).
Fig.5
Taken together, these results indicate that, by varying the ponA concentration, precise control of gene dosage can be achieved with this system. Furthermore, comparatively brief induction periods can result in low to moderate levels of gene expression.
