| Cyclin T1 is the defining regulatory subunit of the CDK9/cyclin T1 complex P‑TEFb, where it directs a cyclin‑dependent kinase module that converts RNA polymerase II from an initiation‑paused state to productive elongation by targeting the C‑terminal domain (CTD) and negative elongation factors. The protein contains an N‑terminal cyclin box fold that wraps around the N‑terminal lobe of CDK9 to stabilise its active conformation, followed by extended low‑complexity regions, including a histidine‑rich intrinsically disordered domain and defined interaction motifs for transcriptional regulators and viral proteins, thereby providing both catalytic activation and targeting functions within elongation complexes. P‑TEFb assembled around cyclin T1 phosphorylates serine‑2 residues in the heptad repeats of the RPB1 CTD and also phosphorylates pausing factors such as NELF and DSIF, which relieves promoter‑proximal pausing and promotes recruitment of RNA‑processing factors, thereby driving the transition into high‑processivity elongation on a broad set of protein‑coding genes. A histidine‑rich low‑complexity domain within cyclin T1 acts as a sequence determinant that augments P‑TEFb function by promoting multivalent interactions with the CTD; this domain enhances binding of P‑TEFb to CTD substrates, stimulates hyperphosphorylation of the CTD by CDK9 in cells, and is necessary for full transcriptional stimulation at P‑TEFb‑dependent loci. The same histidine‑rich domain drives liquid–liquid phase separation of cyclin T1 into dynamic droplets and nuclear speckle‑like condensates, and pre‑phosphorylated CTD is efficiently partitioned into these cyclin T1 condensates, creating a compartment where P‑TEFb and its substrate are concentrated, and CTD hyperphosphorylation and elongation are strongly favored. On CDK9/cyclin T1 core, an interface that positions the CDK9 activation loop and T‑loop in an active configuration and provides docking surfaces for Tat and other factors, explaining how cyclin T1 both stabilizes CDK9 and organizes a platform for recruitment to specific transcription units. Cyclin T1 binds directly to the activation domain of the viral Tat protein and, together with its recognition of the TAR RNA stem–loop in nascent transcripts, forms a Tat–cyclin T1–CDK9 complex that is tethered to the viral LTR, resulting in strong CTD hyperphosphorylation and efficient transcription of full‑length viral RNAs. Dominant‑negative cyclin T1 mutants that disrupt Tat binding or P‑TEFb assembly inhibit Tat‑dependent transactivation and viral replication. |
