Provided herein is a novel assay for the quantification of P-TEFb activation in living cells. The invention, in one embodiment, comprises cells which express P-TEFb and a P-TEFb-phosphorylated or P-TEFb-binding species, for example the C terminal domain of RNA polymerase II. Each of the P-TEFb and P-TEFb-phosphorylated or P-TEFb-binding species comprises a complementary signal moiety, for example complementary fragments of a fluorescent protein, such that when the P-TEFb interacts with the P-TEFb-phosphorylated or P-TEFb-binding species, the signal moieties are in sufficiently close proximity to generate a detectable signal.
Visualization of P-TEFb by Fluorescence Complementation