P-TEFb, a cyclin-dependent kinase controlling elongation by RNA polymeraseII

The elongation phase of transcription by RNA polymerase II is one of the ma ny steps during the generation of mature mRNAs that is subject to regulatio n. Shortly after initiation, RNA polymerase II comes under the control of n egative transcription elongation factors, generally termed N-TEFs, and ente rs abortive elongation (51). During this postinitiation process, only short transcripts are generated that may be prematurely terminated. These short transcripts arise from transcription of many genes, including c-myb, c-myc, c-fos, HSP70, and the human immunodeficiency virus (HIV) long terminal rep eat (LTR), and are normally subject to rapid degradation (3, 63). Escape fr om the action of N-TEF requires the action of at least one positive transcr iption elongation factor (P-TEF), eventually identified as P-TEFb (52). P-T EFb allows the transition into productive elongation, producing long transc ripts from which mRNAs are derived. In this way, the fraction of initiating RNA polymerase II molecules that produce full-length transcripts is contro lled by a selection process that occurs early in the elongation phase of th e transcription cycle. After the transition is made into productive elongat ion, the efficiency of elongation may be influenced by additional factors, including S-II, TFIIF, ELL, and elongin (62, 65).