Imagine you are studying an inducible transcription factor called X. You make a cellular lysate and carry out a series of centrifugation steps at increasing speeds to localize X. You find that prior to induction, X is in the supernatant after a fourth, very high speed ultracentrifugation step; however, after induction it is found in the pellet after only one, relatively low speed ultracentrifugation step. What might this tell you about how X is regulated

X is regulated through nuclear localization

Explanation:

Before induction, the transcription factor X can be localized in the supernatant subsequent to the high-speed ultracentrifugation treatment, showing that X is found in the cytosol. After induction, the protein X can be pelleted subsequent to the low-speed ultracentrifugation treatment, showing nuclear localization. The protein X is a transcription factor, thereby it promotes the transcription of genes in the cell nucleus. Most transcription factors have nuclear localization signals (NLS) that tag the protein transporting them to the cell nucleus. Generally, NLSs are short polypeptides composed of positively charged amino acids (i.e., arginines and/or lysines) exposed on the protein surface.

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