Imagine a Transcription Regulatory Protein (X) That Is Known to Shuttle

Short Answer

Imagine a transcription regulatory protein (X) that is known to shuttle back and forth between nucleus and cytosol in an oscillatory pattern. Protein Y is a nuclear protein that can bind to X to create a dimer that binds to DNA. You have fused protein X to green fluorescent protein (GFP) and protein Y to blue fluorescent protein (BFP), and have measured fluorescence resonance energy transfer (FRET) and non-FRET signals in the nucleus at different time points, as indicated in the following simplified plot. At which time period (1 or 2) do you think protein X is in the nucleus? BFP can be excited at 440 nm, and emits maximally at 470 nm. GFP is excited at 470 nm and emits maximally at 500 nm.