Cyclins are eukaryotic proteins that play an active role in controlling nuclear cell division cycles [], and regulate cyclin dependent kinases (CDKs). This entry represents cyclin-T1 and cyclin-T.Cyclin-T associates with CDK9. Both cyclin-T and CDK9 are subunits of the positive-transcription elongation factor (P-TEFb), which facilitates transcription elongation by phosphorylating the carboxy-terminal domain (CTD) of the largest subunit of pol II []. In humans, there are three T-type cyclins: cyclins T1, T2a and T2b. Cyclins T2a and T2b are splice variants of a primary transcript []. Each of the T-type cyclin/CDK9 complexes can phosphorylate the C-terminal domain of the large subunit of RNA polymerase II. However, only human cyclin T1/CDK9 complexes can bind HIV Tat and promote RNA Pol II activation, allowing transcription of viral genes [, , , ].
