CDK9 has the intrinsic property to shuttle between nucleus and cytoplasm, and enhanced expression of cyclin T1 promotes its nuclear localization.

CDK9 in association with cyclin T constitutes the P-TEFb complex that stimulates transcription elongation of RNAPII transcripts by phosphorylation of the CTD of RNAPII. Here we report subcellular distribution of P-TEFb in terms of localization of CDK9 and cyclin T1. We found that cyclin T1 is exclusively nuclear and it is present in nuclear-speckled structures. CDK9, albeit mainly nuclear, was also visualized in the cytoplasm. We determined that CDK9 is actively exported from the nucleus, and that leptomycin B (LMB), a specific inhibitor of nuclear export, inhibits this process. Interestingly, enforced expression of cyclin T1 enhances nuclear localization of CDK9. These findings reveal a novel control mechanism for the function of the P-TEFb complex. Copyright 2002 Wiley-Liss, Inc.