ABSTRACT: Although histone deacetylases (HDACs) are normally considered as co-repressors, HDAC1 has been identified as a coactivator for the glucocorticoid receptor (GR) (Qiu, Y., Zhao, Y., Becker, M., John, S., Parekh, B. S., Huang, S., Hendarwanto, A., Martinez, E. D., Chen, Y., Lu, H., Adkins, N. L., Stavreva, D. A., Wiench, M., Georgel, P. T., Schiltz, R. L., and Hager, G. L. (2006) Mol. Cell 22, 669-679). Furthermore, HDAC1 is acetylated, and its acetylation level is linked to the transcription state of a GR-induced promoter (mouse mammary tumor virus). GR is also known to interact dynamically with regulatory elements in living cells (McNally, J. G., Müller, W. G., Walker, D., Wolford, R., and Hager, G. L. (2000) Science 287, 1262-1265). However, HDAC1 dynamics have never been studied. We demon