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).