capable of fluorescence resonance energy transfer (FRET). This peptide is known to be a substrate of p300 acetyl-transferase activity (Ott, M. et al. 1999). We shall show that the efficiency of FRET is significantly decreased upon Tat acetylation by p300. Moreover, the choice of this cell-permeable construct allows us to visualize the acetylation states in living cells bypassing cellinvasive procedures. Our results indicate that the sensor can discriminate between basal or altered acetylation

more »

... ates. We shall present results for the case of cells over-expressing p300 or under TSA drug treatment. We shall argue that this method can provide a general approach for screening acetyltransferase activity in live cells. 3014-Pos Quenching of Alexa Dyes by Amino Acids Alexa dyes, rhodamine-derived fluorophores, are popular choices for labeling proteins due to their superior photo-physical properties. They are often used in quantitative fluorescence measurements like Forster Resonance Energy Transfer (FRET) or fluorescence lifetime imaging (FLIM). Consequently, it is important to consider the effects of nearby amino acid residues on the brightness of fluorophores that may influence quantitative measurements of fluorescence intensities or lifetimes. We report on the quenching of Alexa dyes (488, 555 and 594) by various natural amino acids. We observed quenching of