We report the identification of a transcription elongation factor from HeLa cell nuclear extracts that causes pausing of RNA polymerase II (Pol II) in conjunction with the transcription inhibitor 5,6-dichloro-1-␤-D-ribofuranosylbenzimidazole (DRB). This factor, termed DRB sensitivity-inducing factor (DSIF), is also required for transcription inhibition by H8. DSIF has been purified and is composed of 160-kD (p160) and 14-kD (p14) subunits. Isolation of a cDNA encoding DSIF p160 shows it to be a homolog of the Saccharomyces cerevisiae transcription factor Spt5. Recombinant Supt4h protein, the human homolog of yeast Spt4, is functionally equivalent to DSIF p14, indicating that DSIF is composed of the human homologs of Spt4 and Spt5. In addition to its negative role in elongation, DSIF is able to stimulate the rate of elongation by RNA Pol II in a reaction containing limiting concentrations of ribonucleoside triphosphates. A role for DSIF in transcription elongation is further supported by the fact that p160 has a region homologous to the bacterial elongation factor NusG. The combination of biochemical studies on DSIF and genetic analysis of Spt4 and Spt5 in yeast, also in this issue, indicates that DSIF associates with RNA Pol II and regulates its processivity in vitro and in vivo.

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, two 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 transcription. This factor, termed negative elongation factor (NELF), cooperates with DSIF and strongly represses pol II elongation. This repression is reversed by P-TEFb-dependent phosphorylation of the pol II C-terminal domain. NELF 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.

Receptor protein-tyrosine kinases (RPTKs) are critical regulatory enzymes for cell growth, differentiation, and development. Binding of a growth factor to its cognate RPTK promotes receptor dimerization and "trans-phosphorylation" on multiple tyrosine residues (1). These phosphotyrosines serve as docking sites for secondary signaling molecules containing Src homology 2 (SH2) domains, most of which are also RPTK substrates. SH2 domains mediate specific, high-affinity interactions with phosphotyrosine-

The elongation step of RNA polymerase II (RNAPII) transcription is emerging as a critical control point for the expression of various genes and for diverse biological processes. Examples include neuronal fate determination during embryonic development (6, 44), gene expression of human immunodeficiency virus (5,11,13,19,43), replication and transcription of hepatitis delta virus (38), and transcriptional regulation of heat shock genes (1,10,18). In all these cases, the involvement of three transcription elongation factors, namely, DRB (5,6-dichloro-1-␤-D-ribofuranosylbenzimidazole) sensitivity-inducing factor (DSIF), NELF (negative elongation factor), and positive transcription elongation factor b (P-TEFb), has been demonstrated or implicated.Shortly after the initiation of transcription, RNAPII comes under the negative and positive control of DSIF, NELF, and P-TEFb. DSIF and NELF cause transcriptional pausing through physical association with RNAPII. DSIF binds to RNAPII directly and stably (33, 36). However, this appears to have little effect on the catalytic activity of RNAPII (37). A previous study has pointed out that NELF does not bind substantially to DSIF or RNAPII alone but does bind to the complex of DSIF and RNAPII (40). This association is the likely trigger of transcriptional pausing. Conversely, P-TEFb allows RNAPII to enter the productive elongation phase by preventing the action of DSIF and NELF (27, 37). P-TEFb is the protein kinase whose primary target is thought to be the C-terminal domain (CTD) of RNAPII (26). Most, but not all, evidence suggests that P-TEFb-dependent phosphorylation of the CTD facilitates the release of DSIF and NELF from RNA-PII, thereby reversing the inhibition (3, 24, 37). In theory, such regulation at the elongation step allows for rapid change in mRNA levels and for highly sophisticated control over gene expression when combined with regulation at the (pre)initiation step.The structures and functions of DSIF and P-TEFb have been extensively characterized. Human DSIF is a heterodimer composed of p14 (14 kDa) and p160 (160 kDa), whose Saccharomyces cerevisiae counterparts are Spt4 and Spt5 (7,33). In addition to its role in transcriptional pausing, DSIF has a potential to activate RNAPII elongation. The activation mechanism is not well understood: interaction partners of DSIF other than NELF may be involved (13,14,20,23,28). Spt5 has a highly acidic N-terminal region, multiple copies of the KOW motifs, and a repetitive C-terminal region analogous to the RNAPII CTD (9,25,36). RNAPII interacts with Spt5 through a region encompassing the KOW motifs. KOW motifs are also found in the bacterial transcription elongation factor NusG, which binds to prokaryotic RNA polymerase and controls termination and antitermination (15,17,29). In addition, the extreme C terminus of Spt5 is specifically involved in the transcriptional repression pathway (6). Human P-TEFb is a heterodimer composed of Cdk9 (41 kDa) and one of multiple cyclin subunits T1, T2a, T2b, and K (50 to 90 kDa) (26). The k...

Efficient transcription and replication of the influenza virus genome are dependent upon host-derived factors. Using an in vitro RNA synthesis system, we have purified and identified Hsp90 as one of the host factors that stimulate viral RNA polymerase activity. Hsp90 interacted with the PB2 subunit of the viral RNA polymerase through the amino-terminal chaperone domain and the middle region containing a highly acidic domain. The acidic middle region was also responsible for its stimulatory activity. We found that a portion of Hsp90 is re-localized to the cell nucleus after viral infection. A PB2 fragment containing a Hsp90 binding domain inhibited viral gene expression in a dominant-negative manner. These results suggest that Hsp90 is a host factor for the influenza virus RNA polymerase.Influenza A virus belongs to the Orthomyxoviridae family, and its genome consists of eight segmented, single-stranded RNA of negative polarity (1). The transcription promoter and the replication signal of the viral genome exist at the 3Ј and 5Ј termini of each of the eight segments. Components associated with ribonucleoprotein complexes (vRNP) 1 purified from virions are the minimum factors required for primary transcription. The genome RNA forms vRNP with the viral RNA polymerases consisting of three subunits, PB2, PB1, and PA (2), and nucleocapsid protein (NP). Transcription of the influenza virus genome is initiated with host-derived oligo RNA containing a cap structure. PB2 contains cap recognition domains at its carboxyl-terminal region. The capped RNA bound to PB2 is cleaved by the PB1 subunit 10 -15 bases downstream from the 5Ј end (2-4), and the capped RNA fragment serves as a primer for viral mRNA synthesis catalyzed by PB1 (5). Elongation of the RNA chain proceeds until the polymerase reaches a polyadenylation signal consisting of 5-7 uracil (U) residues located near the 5Ј terminal region of the vRNA (6). The viral RNA polymerase polyadenylates the nascent RNA chain possibly by a slippage mechanism at the U-stretch (7). Replication of the vRNA is thought to take place by a primer-independent, twostep reaction, namely the complementary RNAs (cRNA) are first synthesized from vRNA templates, and then the progeny vRNAs are amplified from cRNA templates. Genetic analyses suggest that PA participates in the replication process (8). However, vRNP complexes isolated from virions are incapable of catalyzing replication reactions.A variety of host proteins have been identified as factors involved in the regulation of the RNA synthesis of viral genomes of Paramyxoviridae, the genome of which contains nonsegmented and single-stranded RNA of negative polarity. Tubulin, an acidic cytoplasmic structural protein, is one of the host factors for RNA synthesis of the measles virus, VSV, and Sendai virus genomes (9, 10). RNA synthesis of these viral genomes is catalyzed by viral RNA polymerases consisting of L and P subunits. Tubulin interacts with L protein, a catalytic subunit of the viral RNA polymerase, and is present in isolated tr...