Interaction between calf thymus RNA polymerase II and singly nicked Simian virus 40 DNA

Transcription factors TFIIF, ELL, and Elongin negatively regulate SII-induced nascent transcript cleavage by non-arrested RNA polymerase II elongation intermediates

TFIIF, ELL, and Elongin belong to a class of RNA polymerase II transcription factors that function similarly to activate the rate of elongation by suppressing transient pausing by polymerase at many sites along DNA templates. SII is a functionally distinct RNA polymerase II elongation factor that promotes elongation by reactivating arrested polymerase. Studies of the mechanism of SII action have shown (i) that arrest of RNA polymerase II results from irreversible displacement of the 3'-end of the nascent transcript from the polymerase catalytic site and (ii) that SII reactivates arrested polymerase by inducing endonucleolytic cleavage of the nascent transcript by the polymerase catalytic site thereby creating a new transcript 3'-end that is properly aligned with the catalytic site and can be extended. SII also induces nascent transcript cleavage by paused but non-arrested RNA polymerase II elongation intermediates, leading to the proposal that pausing may result from reversible displacement of the 3'-end of nascent transcripts from the polymerase catalytic site. On the basis of evidence consistent with the model that TFIIF, ELL, and Elongin suppress pausing by preventing displacement of the 3'-end of the nascent transcript from the polymerase catalytic site, we investigated the possibility of cross-talk between SII and transcription factors TFIIF, ELL, and Elongin. These studies led to the discovery that TFIIF, ELL, and Elongin are all capable of inhibiting SII-induced nascent transcript cleavage by non-arrested RNA polymerase II elongation intermediates. Here we present these findings, which bring to light a novel activity associated with TFIIF, ELL, and Elongin and suggest that these transcription factors may expedite elongation not only by increasing the forward rate of nucleotide addition by RNA polymerase II, but also by inhibiting SII-induced nascent transcript cleavage by non-arrested RNA polymerase II elongation intermediates.

Transcription elongation and human disease

Eukaryotic mRNA synthesis is catalyzed by multisubunit RNA polymerase II and proceeds through multiple stages referred to as preinitiation, initiation, elongation, and termination. Over the past 20 years, biochemical studies of eukaryotic mRNA synthesis have largely focused on the preinitiation and initiation stages of transcription. These studies led to the discovery of the class of general initiation factors (TFIIB, TFIID, TFIIE, TFIIF, and TFIIH), which function in intimate association with RNA polymerase II and are required for selective binding of polymerase to its promoters, formation of the open complex, and synthesis of the first few phosphodiester bonds of nascent transcripts. Recently, biochemical studies of the elongation stage of eukaryotic mRNA synthesis have led to the discovery of several cellular proteins that have properties expected of general elongation factors and that have been found to play unanticipated roles in human disease. Among these candidate general elongation factors are the positive transcription elongation factor b (P-TEFb), eleven-nineteen lysine-rich in leukemia (ELL), Cockayne syndrome complementation group B (CSB), and elongin proteins, which all function in vitro to expedite elongation by RNA polymerase II by suppressing transient pausing or premature arrest by polymerase through direct interactions with the elongation complex. Despite their similar activities in elongation, the P-TEFb, ELL, CSB, and elongin proteins appear to play roles in a diverse collection of human diseases, including human immunodeficiency virus-1 infection, acute myeloid leukemia, Cockayne syndrome, and the familial cancer predisposition syndrome von Hippel-Lindau disease. here we review our current understanding of the P-TEFb, ELL, CSB, and elongin proteins, their mechanisms of action, and their roles in human disease.

Dual roles for transcription factor IIF in promoter escape by RNA polymerase II

Transcription factor (TF) IIF is a multifunctional RNA polymerase II transcription factor that has well established roles in both transcription initiation, where it functions as a component of the preinitiation complex and is required for formation of the open complex and synthesis of the first phosphodiester bond of nascent transcripts, and in transcription elongation, where it is capable of interacting directly with the ternary elongation complex and stimulating the rate of transcription. In this report, we present evidence that TFIIF is also required for efficient promoter escape by RNA polymerase II. Our findings argue that TFIIF performs dual roles in this process. We observe (i) that TFIIF suppresses the frequency of abortive transcription by very early RNA polymerase II elongation intermediates by increasing their processivity and (ii) that TFIIF cooperates with TFIIH to prevent premature arrest of early elongation intermediates. In addition, our findings argue that two TFIIF functional domains mediate TFIIF action in promoter escape. First, we observe that a TFIIF mutant selectively lacking elongation activity supports TFIIH action in promoter escape, but is defective in suppressing the frequency of abortive transcription by very early RNA polymerase II elongation intermediates. Second, a TFIIF mutant selectively lacking initiation activity is more active than wild type TFIIF in increasing the processivity of very early elongation intermediates, but is defective in supporting TFIIH action in promoter escape. Taken together, our findings bring to light a function for TFIIF in promoter escape and support a role for TFIIF elongation activity in this process.

Mechanism of action of RNA polymerase II elongation factor Elongin. Maximal stimulation of elongation requires conversion of the early elongation complex to an Elongin-activable form

We previously identified and purified Elongin by its ability to stimulate the rate of elongation by RNA polymerase II in vitro (Bradsher, J. N., Jackson, K. W., Conaway, R. C., and Conaway, J. W. (1993) J. Biol. Chem. 268, 25587-25593). In this report, we present evidence that stimulation of elongation by Elongin requires that the early RNA polymerase II elongation complex undergoes conversion to an Elongin-activable form. We observe (i) that Elongin does not detectably stimulate the rate of promoter-specific transcription initiation by the fully assembled preinitiation complex and (ii) that early RNA polymerase II elongation intermediates first become susceptible to stimulation by Elongin after synthesizing 8-9-nucleotide-long transcripts. Furthermore, we show that the relative inability of Elongin to stimulate elongation by early elongation intermediates correlates not with the lengths of their associated transcripts but, instead, with the presence of transcription factor IIF (TFIIF) in transcription reactions. By exploiting adenovirus 2 major late promoter derivatives that contain premelted transcriptional start sites and do not require TFIIF, TFIIE, or TFIIH for transcription initiation, we observe (i) that Elongin is capable of strongly stimulating the rate of synthesis of trinucleotide transcripts by a subcomplex of RNA polymerase II, TBP, and TFIIB and (ii) that the ability of Elongin to stimulate synthesis of these short transcripts is substantially reduced by addition of TFIIF to transcription reactions. Here we present these findings, which are consistent with the model that maximal stimulation of elongation by Elongin requires that early elongation intermediates undergo a structural transition that includes loss of TFIIF.

A novel activity associated with RNA polymerase II elongation factor SIII. SIII directs promoter-independent transcription initiation by RNA polymerase II in the absence of initiation factors

General transcription factor SIII, a heterotrimer of 110-, 18-, and 15-kDa subunits, was shown previously to stimulate the overall rate of RNA chain elongation by RNA polymerase II by suppressing transient pausing by polymerase at many sites along DNA templates (Bradsher, J. N., Jackson, K. W., Conaway, R. C., and Conaway, J. W. (1993) J. Biol. Chem. 268, 25587-25593). In this report, SIII is shown to possesses the novel ability to direct robust but promiscuous transcription by RNA polymerase II on duplex DNA templates in the absence of initiation factors. Mechanistic studies reveal that SIII promotes RNA synthesis by substantially increasing the efficiency with which RNA polymerase II initiates promoter-independent transcription from the ends of duplex DNA. Remarkably, SIII appears to have a negligible effect on de novo synthesis of end-to-end transcripts. Instead, analysis of reaction products indicates that SIII is capable of promoting a dramatic increase in the ability of RNA polymerase II to extend the 3'-hydroxyl termini of duplex DNA fragments, in a template-directed reaction exhibiting no strong preference for 3'-protruding, 3'-recessed, or blunt DNA ends. Although RNA polymerase II has been shown previously to catalyze primer-dependent transcription, SIII is the first eukaryotic transcription factor found to promote this reaction. Based on these findings, we propose that SIII may suppress transient pausing by RNA polymerase II by helping to maintain the 3'-hydroxyl terminus of the nascent RNA chain in its proper position in the polymerase active site.

Analysis of the signals for transcription termination by purified RNA polymerase II

Eukaryotic RNA polymerase II recognizes certain DNA sequences as effective signals for transcription termination in vitro. Previously, we have shown that such termination occurs within T-rich sequences; however, not all T runs stop the enzyme nor is the efficiency of termination correlated with the length of the T run. Here we have investigated the sequence elements that signal transcription termination by purified RNA polymerase II. We have examined terminators located within introns of the human histone H3.3 gene and the human c-myc gene. Deletion analysis of the H3.3 termination region indicates that the sequences between -6 and +24 relative to the strongest termination site are sufficient to cause transcription termination. The minimal termination signal at this site has been localized to the sequence TTTTTTTC-CCTTTTTT in the nontranscribed strand. A similar but nonidentical sequence has been defined for the c-myc termination site. Since RNA polymerase II terminates transcription only within the first run of T residues in these sequences, at least part of the termination signal lies in downstream nontranscribed DNA sequences. Restriction fragment mobility analysis indicates that the H3.3 termination region contains a bend in the DNA helix. Oligonucleotides containing the minimal termination signals also cause restriction fragments to migrate with anomalous mobility. A region of the SV40 genome containing a previously characterized bend also causes RNA polymerase II to terminate transcription. We suggest that a structural element causing a bend in the DNA helix may be part of the signal for transcription termination by purified RNA polymerase II.

Renaturase and ribonuclease H: a novel mechanism that influences transcript displacement by RNA polymerase II in vitro

I have previously reported an activity in HeLa cells which facilitates transcript displacement by purified mammalian RNA polymerase II in vitro. I have shown that this activity copurifies with one of two separable ribonuclease (RNase) H activities in HeLa cells. The RNase H activity in question has characteristics similar to those reported for RNase H2b from calf thymus. RNase H proteins purified from several other sources including Escherichia coli also show renaturase activity. When the renaturase/RNase H protein is present during transcription by purified RNA polymerase II, transcripts are truncated close to the 5' end, and the remainder of the transcript is displaced normally from its template by the polymerase. Since RNA polymerase II dependent transcripts in vivo normally require the presence of the 5'-triphosphate terminus for capping, the in vivo significance of RNase H as a renaturase factor is presently not understood. However, the in vitro action of renaturase/RNase H suggests that the mechanism of this reaction may involve R-loop displacement after formation of a short single-stranded region of DNA on the template strand following hydrolysis of a hybrid transcript oligonucleotide by RNase H.

Functional analysis of reverse transcription by a frameshift pol mutant of murine leukemia virus

Endogenous reverse transcription by wild-type murine leukemia virus (MuLV) was compared to that catalyzed by clone 23, a pol mutant containing a reverse transcriptase protein which lacks the carboxyl-terminal third of the molecule (J. G. Levin, S. C. Hu, A. Rein, L. I. Messer, and B. I. Gerwin (1984), J. Virol. 51, 470-478). Competition immunoassays revealed that mutant virions contain normal amounts of polymerase protein, indicating that the lack of carboxyl-terminal sequences does not alter normal processing of enzyme precursors. Although the mutant enzyme was previously shown to have the ability to copy and degrade RNA:DNA hybrids, the present study demonstrates that it is defective in functions required to generate full-length copies of viral DNA. Analysis of products of endogenous reverse transcription showed that minus-strand strong-stop DNA is formed and that mutant virions synthesize a series of minus-strand DNA intermediates up to 2.2 kb in length. Comparison of mutant and wild-type MuLV reaction products indicated that the 2.2-kb termination site of the mutant corresponds to a normal pausing region for the wild-type enzyme. Computer analysis of sequences and structure within pausing regions suggested the involvement of C-rich consensus sequences plus multibranch loop structures in the general phenomenon of enzyme-pausing during reverse transcription.

Eukaryotic RNA polymerases

This review will attempt to cover the present information on the multiple forms of eukaryotic DNA-dependent RNA polymerases, both at the structural and functional level. Nuclear RNA polymerases constitute a group of three large multimeric enzymes, each with a different and complex subunit structure and distinct specificity. The review will include a detailed description of their molecular structure. The current approaches to elucidate subunit function via chemical modification, phosphorylation, enzyme reconstitution, immunological studies, and mutant analysis will be described. In vitro reconstituted systems are available for the accurate transcription of cloned genes coding for rRNA, tRNA, 5 SRNA, and mRNA. These systems will be described with special attention to the cellular factors required for specific transcription. A section on future prospects will address questions concerning the significance of the complex subunit structure of the nuclear enzymes; the organization and regulation of the gene coding for RNA polymerase subunits; the obtention of mutants affected at the level of factors, or RNA polymerases; the mechanism of template recognition by factors and RNA polymerase.