Regulation of two key nuclear enzymatic activities by the 7SK small nuclear RNA

7SK is a highly conserved small nuclear RNA (snRNA) in vertebrates. Since its discovery in 1968, little had been known about its function until recently, when 7SK was found to associate with the general transcription elongation factor P-TEFb. Together with the HEXIM1 protein, 7SK sequesters P-TEFb into a kinase-inactive complex, where it mediates HEXIM1's inhibition of P-TEFb. This helps maintain P-TEFb in a functional equilibrium to control transcription, cell growth, and differentiation. Although highly abundant, only a small fraction of 7SK is P-TEFb-bound. Using affinity purification, we have identified APOBEC3C as another 7SK-associated protein. As a member of the APOBEC family that functions in diverse processes through deaminating cytosine in DNA, it is unclear how APOBEC3C's activity is controlled to prevent its mutations of genomic DNA. We show that most of APOBEC3C interact with about half of nuclear 7SK, which suppresses APOBEC3C's deaminase activity and sequesters APOBEC3C in the nucleolus where it could be at a safe distance from most genomic sequences. Because the DNA substrate-binding site in APOBEC3C differs from the region for 7SK binding, 7SK does not act as a substrate competitor in inhibiting APOBEC3C. The demonstration of 7SK's suppression of yet another enzyme besides P-TEFb suggests a general role for this RNA in regulating key nuclear functions.