Mechanism of eIF6 release from the nascent 60S ribosomal subunit.

SBDS protein (deficient in the inherited leukemia-predisposition disorder Shwachman-Diamond syndrome) and the GTPase EFL1 (an EF-G homolog) activate nascent 60S ribosomal subunits for translation by catalyzing eviction of the antiassociation factor eIF6 from nascent 60S ribosomal subunits. However, the mechanism is completely unknown. Here, we present cryo-EM structures of human SBDS and SBDS-EFL1 bound to Dictyostelium discoideum 60S ribosomal subunits with and without endogenous eIF6. SBDS assesses the integrity of the peptidyl (P) site, bridging uL16 (mutated in T-cell acute lymphoblastic leukemia) with uL11 at the P-stalk base and the sarcin-ricin loop. Upon EFL1 binding, SBDS is repositioned around helix 69, thus facilitating a conformational switch in EFL1 that displaces eIF6 by competing for an overlapping binding site on the 60S ribosomal subunit. Our data reveal the conserved mechanism of eIF6 release, which is corrupted in both inherited and sporadic leukemias.

Aberrant 3' oligoadenylation of spliceosomal U6 small nuclear RNA in poikiloderma with neutropenia.

The recessive disorder poikiloderma with neutropenia (PN) is caused by mutations in the C16orf57 gene that encodes the highly conserved USB1 protein. Here, we present the 1.1 Å resolution crystal structure of human USB1, defining it as a member of the LigT-like superfamily of 2H phosphoesterases. We show that human USB1 is a distributive 3'-5' exoribonuclease that posttranscriptionally removes uridine and adenosine nucleosides from the 3' end of spliceosomal U6 small nuclear RNA (snRNA), directly catalyzing terminal 2', 3' cyclic phosphate formation. USB1 measures the appropriate length of the U6 oligo(U) tail by reading the position of a key adenine nucleotide (A102) and pausing 5 uridine residues downstream.We show that the 3' ends of U6 snRNA in PN patient lymphoblasts are elongated and unexpectedly carry nontemplated 3' oligo(A) tails that are characteristic of nuclear RNA surveillance targets. Thus, our study reveals a novel quality control pathway in which posttranscriptional 3'-end processing by USB1 protects U6 snRNA from targeting and destruction by the nuclear exosome. Our data implicate aberrant oligoadenylation of U6 snRNA in the pathogenesis of the leukemia predisposition disorder PN.

The DNA sequence-dependence of nucleosome positioning in vivo and in vitro.

The contribution of histone-DNA interactions to nucleosome positioning in vivo is currently a matter of debate. We argue here that certain nucleosome positions, often in promoter regions, in yeast may be, at least in part, specified by the DNA sequence. In contrast other positions may be poorly specified. Positioning thus has both statistical and DNA-determined components. We further argue that the relative affinity of the octamer for different DNA sequences can vary and therefore the interaction of histones with the DNA is a 'tunable' property.

Nucleosome positioning--what do we really know?

The positioning of nucleosomes on the DNA of eukaryotic genomes is a major determinant of gene expression. In particular nucleosomes in close proximity to regulatory regions are often more precisely positioned in vivo than nucleosomes located elsewhere. In this article we compare data obtained from the most recent studies by a variety of techniques. We argue that the disparate conclusions in the literature could be a consequence of procedural differences sampling alternative arrays of nucleosomes on the same DNA sequence. Importantly, the ostensibly least invasive techniques identify differences between nucleosomes in the vicinity of transcription start sites in budding yeast and those positioned distally within the transcribed region.

A translational signature for nucleosome positioning in vivo.

In vivo nucleosomes often occupy well-defined preferred positions on genomic DNA. An important question is to what extent these preferred positions are directly encoded by the DNA sequence itself. We derive here from in vivo positions, accurately mapped by partial micrococcal nuclease digestion, a translational positioning signal that identifies the approximate midpoint of DNA bound by a histone octamer. This midpoint is, on average, highly A/T rich ( approximately 73%) and, in particular, the dinucleotide TpA occurs preferentially at this and other outward-facing minor grooves. We conclude that in this set of sequences the sequence code for DNA bending and nucleosome positioning differs from the other described sets and we suggest that the enrichment of AT-containing dinucleotides at the centre is required for local untwisting. We show that this signature is preferentially associated with nucleosomes flanking promoter regions and suggest that it contributes to the establishment of gene-specific nucleosome arrays.

RNA-protein crosslink mapping using TEV protease.

Characterization of novel RNA-protein interactions often demands physical mapping of the RNA binding sites in the protein. This can sometimes be accomplished using radioactively labeled RNA in covalent RNA-protein crosslinking experiments. The position of the radioactive label crosslinked to the protein can then be determined by fragmentation of the protein using a battery of sequence-specific proteolytic enzymes or chemical reagents. However, there are typically many cleavage sites in the natural protein sequence, and for large proteins, particularly when there are multiple sites of RNA-protein interaction, it may be difficult or impossible to determine the sites of crosslink formation unambiguously using this traditional physical mapping approach. We have developed an alternative method for physical mapping of RNA-protein crosslinks based on random insertion into the protein of a short peptide tag that includes the target sequence ENLYFQG (Glu-Asn-Leu-Tyr-Phe-Gln-Gly) for the highly specific TEV protease from tobacco etch virus. Covalent RNA-protein crosslinks can then be physically mapped by TEV protease digestion, fractionation of the proteolytic digestion products by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and visualization of the labeled protein fragments by phosphorimaging.

Dissection of Prp8 protein defines multiple interactions with crucial RNA sequences in the catalytic core of the spliceosome.

Current models of the core of the spliceosome include a network of RNA-RNA interactions involving the pre-mRNA and the U2, U5, and U6 snRNAs. The essential spliceosomal protein Prp8 interacts with U5 and U6 snRNAs and with specific pre-mRNA sequences that participate in catalysis. This close association with crucial RNA sequences, together with extensive genetic evidence, suggests that Prp8 could directly affect the function of the catalytic core, perhaps acting as a splicing cofactor. However, the sequence of Prp8 is almost entirely novel, and it offers few clues to the molecular basis of Prp8-RNA interactions. We have used an innovative transposon-based strategy to establish that catalytic core RNAs make multiple contacts in the central region of Prp8, underscoring the intimate relationship between this protein and the catalytic center of the spliceosome. Our analysis of RNA interactions identifies a discrete, highly conserved region of Prp8 as a prime candidate for the role of cofactor for the spliceosome's RNA core.

Phosphorylation of the RNA polymerase II carboxyl-terminal domain by CDK9 is directly responsible for human immunodeficiency virus type 1 Tat-activated transcriptional elongation.

Stimulation of transcriptional elongation by the human immunodeficiency virus type 1 Tat protein is mediated by CDK9, a kinase that phosphorylates the RNA polymerase II carboxyl-terminal domain (CTD). In order to obtain direct evidence that this phosphorylation event can alter RNA polymerase processivity, we prepared transcription elongation complexes that were arrested by the lac repressor. The CTD was then dephosphorylated by treatment with protein phosphatase 1. The dephosphorylated transcription complexes were able to resume the transcription elongation when IPTG (isopropyl-beta-D-thiogalactopyranoside) and nucleotides were added to the reaction. Under these chase conditions, efficient rephosphorylation of the CTD was observed in complexes containing the Tat protein but not in transcription complexes prepared in the absence of Tat protein. Immunoblots and kinase assays with synthetic peptides showed that Tat activated CDK9 directly since the enzyme and its cyclin partner, cyclin T1, were present at equivalent levels in transcription complexes prepared in the presence or absence of Tat. Chase experiments with the dephosphorylated elongation transcription complexes were performed in the presence of the CDK9 kinase inhibitor DRB (5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole). Under these conditions there was no rephosphorylation of the CTD during elongation, and transcription through either a stem-loop terminator or bent DNA arrest sequence was strongly inhibited. In experiments in which the CTD was phosphorylated prior to elongation, the amount of readthrough of the terminator sequences was proportional to the extent of the CTD modification. The change in processivity is due to CTD phosphorylation alone, since even after the removal of Spt5, the second substrate for CDK9, RNA polymerase elongation is enhanced by Tat-activated CDK9 activity. We conclude that phosphorylation of the RNA polymerase II CTD by CDK9 enhances transcription elongation directly.

Spt5 cooperates with human immunodeficiency virus type 1 Tat by preventing premature RNA release at terminator sequences.

The human immunodeficiency virus type 1 (HIV-1) Tat protein activates transcription elongation by stimulating the Tat-activated kinase (TAK/p-TEFb), a protein kinase composed of CDK9 and its cyclin partner, cyclin T1. CDK9 is able to hyperphosphorylate the carboxyl-terminal domain (CTD) of the large subunit of RNA polymerase during elongation. In addition to TAK, the transcription elongation factor Spt5 is required for the efficient activation of transcriptional elongation by Tat. To study the role of Spt5 in HIV transcription in more detail, we have developed a three-stage Tat-dependent transcription assay that permits the isolation of active preinitiation complexes, early-stage elongation complexes, and Tat-activated elongation complexes. Spt5 is recruited in the transcription complex shortly after initiation. After recruitment of Tat during elongation through the transactivation response element RNA, CDK9 is activated and induces hyperphosphorylation of Spt5 in parallel to the hyperphosphorylation of the CTD of RNA polymerase II. However, immunodepletion experiments demonstrate that Spt5 is not required for Tat-dependent activation of the kinase. Chase experiments using the Spt5-depleted extracts demonstrate that Spt5 is not required for early elongation. However, Spt5 plays an important role in late elongation by preventing the premature dissociation of RNA from the transcription complex at terminator sequences and reducing the amount of polymerase pausing at arrest sites, including bent DNA sequences. This novel biochemical function of Spt5 is analogous to the function of NusG, an elongation factor found in Escherichia coli that enhances RNA polymerase stability on templates and shows sequence similarity to Spt5.