Lariat sequencing in a unicellular yeast identifies regulated alternative splicing of exons that are evolutionarily conserved with humans.

Alternative splicing is a potent regulator of gene expression that vastly increases proteomic diversity in multicellular eukaryotes and is associated with organismal complexity. Although alternative splicing is widespread in vertebrates, little is known about the evolutionary origins of this process, in part because of the absence of phylogenetically conserved events that cross major eukaryotic clades. Here we describe a lariat-sequencing approach, which offers high sensitivity for detecting splicing events, and its application to the unicellular fungus, Schizosaccharomyces pombe, an organism that shares many of the hallmarks of alternative splicing in mammalian systems but for which no previous examples of exon-skipping had been demonstrated. Over 200 previously unannotated splicing events were identified, including examples of regulated alternative splicing. Remarkably, an evolutionary analysis of four of the exons identified here as subject to skipping in S. pombe reveals high sequence conservation and perfect length conservation with their homologs in scores of plants, animals, and fungi. Moreover, alternative splicing of two of these exons have been documented in multiple vertebrate organisms, making these the first demonstrations of identical alternative-splicing patterns in species that are separated by over 1 billion y of evolution.

The rate of TRAP binding to RNA is crucial for transcription attenuation control of the B. subtilis trp operon.

The trp RNA-binding attenuation protein (TRAP) regulates expression of the tryptophan biosynthetic and transport genes in Bacillus subtilis in response to changes in the levels of intracellular tryptophan. Transcription of the trpEDCFBA operon is controlled by an attenuation mechanism involving two overlapping RNA secondary structures in the 5' leader region of the trp transcript; TRAP binding promotes formation of a transcription terminator structure that halts transcription prior to the structural genes. TRAP consists of 11 identical subunits and is activated to bind RNA by binding up to 11 molecules of L-tryptophan. The TRAP binding site in the leader region of the trp operon mRNA consists of 11 (G/U)AG repeats. We examined the importance of the rate of TRAP binding to RNA for the transcription attenuation mechanism. We compared the properties of two types of TRAP 11-mers: homo-11-mers composed of 11 wild-type subunits, and hetero-11-mers with only one wild-type subunit and ten mutant subunits defective in binding either RNA or tryptophan. The hetero-11-mers bound RNA with only slightly diminished equilibrium binding affinity but with slower on-rates as compared to WT TRAP. The hetero-11-mers showed significantly decreased ability to induce transcription termination in the trp leader region when examined using an in vitro attenuation system. Together these results indicate that the rate of TRAP binding to RNA is a crucial factor in TRAP's ability to control attenuation.

TROSY-NMR studies of the 91kDa TRAP protein reveal allosteric control of a gene regulatory protein by ligand-altered flexibility.

The tryptophan biosynthesis genes of several Bacilli are controlled through terminator/anti-terminator transcriptional attenuation. This process is regulated by tryptophan-dependent binding of the trp RNA-binding attenuation protein (TRAP) to the leader region of the trp operon mRNA, precluding formation of the antiterminator RNA hairpin, and allowing formation of the less stable terminator hairpin. Crystal structures are available of TRAP in complex with tryptophan and in ternary complex with tryptophan and RNA. However, no structure of TRAP in the absence of tryptophan is available; thus, the mechanism of allostery remains unclear. We have used transverse relaxation optimized spectroscopy (TROSY)-based NMR experiments to study the mechanism of ligand-mediated allosteric regulation in the 90.6kDa 11-mer TRAP. By recording a series of two-dimensional 15N-edited TROSY NMR spectra of TRAP from the thermophile Bacillus stearothermophilus over an extended range of temperatures, we have found tryptophan binding to be temperature-dependent, in agreement with the previously observed temperature-dependent RNA binding. Triple-resonance TROSY-based NMR spectra recorded at 55 degrees C have allowed us to obtain backbone resonance assignments for uniformly 2H,13C,15N-labeled TRAP in the inactive form and in the active form (free and bound to tryptophan). On the basis of ligand-dependent differential line-broadening and chemical shift perturbations, coupled with the results of proteolytic sensitivity measurements, we infer that tryptophan-modulated protein flexibility (dynamics) plays a central role in TRAP function by altering its RNA-binding affinity. Furthermore, because the crystal structures show that the ligand is buried completely in the bound state, we speculate that such dynamic behavior may be important to enable rapid response to changes in intracellular tryptophan levels. Thus, we propose that allosteric control of TRAP is accomplished by ligand-altered protein dynamics.

The interaction of RNA with TRAP: the role of triplet repeats and separating spacer nucleotides.

The trp RNA-binding attenuation protein (TRAP) regulates expression of the tryptophan biosynthetic genes in several Bacilli by binding to the leader region of the nascent trp mRNA, inhibiting continued transcription. The 11 subunit TRAP molecule is active in complex with tryptophan, and binds consequently an RNA target segment consisting of 11 (G/U)AG triplets, each separated by two or three non-conserved "spacer" nucleotides. Here, we report the first crystal structures of TRAP in a complex with RNA containing UAG triplets separated by two nucleotides and in a complex with RNA containing GAG triplets separated by three nucleotides. Comparison with known structures of TRAP-RNA complexes shows that both substitution of G-1 with U-1 in the triplet and addition of an extra spacer nucleotide lead to a more flexible complex. This suggests an explanation why, in the trp leader RNA, all three-nucleotide spacer regions are followed by a G-1 nucleotide. Taken together, the structures demonstrate that RNA binding to TRAP is mediated by specific interactions involving the A-2 and G-3 nucleotides of the triplet. This is accompanied by the disruption of stacking interactions between the bases of the other nucleotides, contributing to the increase in entropy that drives binding.

Thermodynamics of tryptophan-mediated activation of the trp RNA-binding attenuation protein.

The trp RNA-binding attenuation protein (TRAP) functions in many bacilli to control the expression of the tryptophan biosynthesis genes. Transcription of the trp operon is controlled by TRAP through an attenuation mechanism, in which competition between two alternative secondary-structural elements in the 5' leader sequence of the nascent mRNA is influenced by tryptophan-dependent binding of TRAP to the RNA. Previously, NMR studies of the undecamer (11-mer) suggested that tryptophan-dependent control of RNA binding by TRAP is accomplished through ligand-induced changes in protein dynamics. We now present further insights into this ligand-coupled event from hydrogen/deuterium (H/D) exchange analysis, differential scanning calorimetry (DSC), and isothermal titration calorimetry (ITC). Scanning calorimetry showed tryptophan dissociation to be independent of global protein unfolding, while analysis of the temperature dependence of the binding enthalpy by ITC revealed a negative heat capacity change larger than expected from surface burial, a hallmark of binding-coupled processes. Analysis of this excess heat capacity change using parameters derived from protein folding studies corresponds to the ordering of 17-24 residues per monomer of TRAP upon tryptophan binding. This result is in agreement with qualitative analysis of residue-specific broadening observed in TROSY NMR spectra of the 91 kDa oligomer. Implications for the mechanism of ligand-mediated TRAP activation through a shift in a preexisting conformational equilibrium and an induced-fit conformational change are discussed.