NELF, a multisubunit complex containing RD, cooperates with DSIF to repress RNA polymerase II elongation

DRB is a classic inhibitor of transcription elongation by RNA polymerase II (pol II). Since DRB generally affects class II genes, factors involved in this process must play fundamental roles in pol II elongation. Recently, tw o elongation factors essential for DRB action were identified, namely DSIF and P-TEFb. Here we describe the identification and purification from HeLa nuclear extract of a third protein factor required for DRB-sensitive transc ription. This factor, termed negative elongation factor (NELF), cooperates with DSIF and strongly represses pol II elongation. This repression is reve rsed by P-TEFb-dependent phosphorylation of the pol II C-terminal domain. N ELF is composed of five polypeptides, the smallest of which is identical to RD, a putative RNA-binding protein of unknown function. This study reveals a molecular mechanism for DRB action and a regulatory network of positive and negative elongation factors.