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We have examined multiple cofactor usage by yeast tRNA ligase in splicing in vitro. The ligase mechanism of action requires expenditure of two molar equivalents of nucleotide cofactor per mole of tRNA product. Recent evidence (Westaway, S.K., Belford, H.G., Apostol, B.L., Abelson, J., and Greer, C.L. (1993) J. Biol. Chem. 268, 2435-2443) demonstrated that the ligase-associated kinase activity is more efficient with GTP as cofactor than with ATP. Employing a ligase fusion construct with dihydrofolate reductase (Apostol, B.L., Westaway, S.K., Abelson, J., and Greer, C.L. (1991) J. Biol. Chem. 266, 7445-7455) for purposes of enzyme purification, we performed joining assays demonstrating that ATP and GTP are the most effective combination of cofactors. ATP was essential to the joining reaction, while UTP, CTP, or ATP replaced GTP inefficiently. Specific and functionally independent binding sites were confirmed for ATP and GTP by direct binding measurement. A third site was implicated in UTP- and CTP-ligase interactions. Comparison of binding constants with Kapp values determined for nucleotide-dependent joining suggested both that nucleotide triphosphate binding may be limiting in tRNA joining and that tRNA ligation occurs most efficiently using GTP for the kinase reaction and ATP as the adenylylate synthetase cofactor.

Yeast tRNA ligase possesses multiple activities which are required for the joining of tRNA halves during the tRNA splicing process: cyclic phosphodiesterase, kinase, adenylylate synthetase, and ligase. A deletion polypeptide of a dihydrofolate reductase-ligase fusion protein, designated DAC, was previously shown to join tRNA halves although ATP-dependent kinase activity was not measurable in the assay used. We describe here a characterization of the mechanism of joining used by DAC and the structure of the tRNA product. DAC produces a joined tRNA and a splice junction with a structure identical to that produced by DAKC, the full-length dihydrofolate reductase-ligase fusion. Furthermore, DAC can use GTP as the sole cofactor in the joining reaction, in contrast to DAKC, which can only complete splicing in the presence of ATP. Both enzymes exhibit GTP-dependent kinase activity at 100-fold greater efficiency than with ATP. These results suggest that a potential function for the center domain of tRNA ligase (missing in DAC) is to provide structural integrity and aid in substrate interactions and specificity. They also support the hypothesis that ligase may prefer to use two different cofactors during tRNA splicing.

Splicing of tRNA precursors in extracts of Saccharomyces cerevisiae requires the action of two enzymes: a site specific endonuclease and a tRNA ligase. The tRNA ligase contains three distinct enzymatic activities: a polynucleotide kinase, a cyclic phosphodiesterase, and an RNA ligase. The polypeptide also has a high affinity pre-tRNA binding site based on its ability to form stable complexes with pre-tRNA substrates. To investigate the organization of functional enzymatic and binding elements within the polypeptide a series of defined tRNA ligase gene deletions were constructed and corresponding proteins were expressed in Escherichia coli as fusions with bacterial dihydrofolate reductase (DHFR). The DHFR/ligase derivative proteins were then efficiently purified by affinity chromatography. The complete ligase fusion protein retained enzymatic and binding activities which were unaffected by the presence of the DHFR segment. Examination of tRNA ligase deletion derivatives revealed that the amino-terminal region was required for adenylylation, while the carboxyl-terminal region was sufficient for cyclic phosphodiesterase activity. Deletions within the central region affected kinase activity. Pre-tRNA binding activity was not strictly correlated with a distinct enzymatic domain. A DHFR/ligase-derived protein lacking kinase activity efficiently joined tRNA halves. We postulate that this variant utilizes a novel RNA ligation mechanism.

We have examined the substrate requirements for efficient and accurate splicing of tRNA precursors in Saccharomyces cerevisiae. The effects of Schizosaccharomyces pombe tRNASer gene mutations on the two steps in splicing, intron excision and joining of tRNA halves, were determined independently by using partially purified splicing endonuclease and tRNA ligase from S. cerevisiae. Two mutations (G14 and A46) reduced the efficiency of excision and joining in parallel, whereas two others (U47:7 and C33) produced differential effects on these two steps; U47:7 affected primarily the excision reaction, and C33 had a greater impact on ligation. These data indicate that endonuclease and ligase recognize both common and unique features of their substrates. Another two mutations (Ai26 and A37:13) induced miscutting, although with converse effects on the two splice sites. Thus, the two cutting events appear to be independent. Finally, we suggest that splice sites may be determined largely through their position relative to sites within the tRNA-like domain of the precursors. Several of these important sites were identified, and others are proposed based on the data described here.

The state vector of a planetary artificial satellite is determined by using Earth-based rangerate measurements. The satellite velocity component in the direction from the planet center to Earth center instead of that from the satellite to an observation station is computed in a theoretical model. The relatively simple, least-squares estimation criterion thus obtained for the case of a planet at infinite distance facilitates the comparison study of numerical methods of solving a system of nonlinear equations. Simulation results obtained by programing in double precision show that the longitude of the ascending node of a planetary satellite can be determined to prescribed accuracy within a few days of tracking. The comparison study is made among 1) classical differential correction method, 2) Newton-Raphson method, 3) generalized differential correction method, and 4) generalized Newton-Raphson method. It indicates that the new generalized differential correction method has a convergence range of initial estimate wider than the other methods. The extension, in the convergence range of initial estimate, enhances the success of obtaining a preliminary state vector in a short tracking period and is particularly important in planetary missions. The effects of perturbations of noncentral forces on the satellite can be incorporated in the formulation without resorting to numerical integration.

A new technique is presented for determining neutron spectra from foil activation data. The set of activation integral equations are replaced by an underdetermined system of linear equations whose solutions are piecewise linear representations of the differential flux. The technique then selects that solution which is closest to a trial solution in a least squares sense. Examples are presented showing various neutron spectra determined from activation data and certain radiation effects computed with these resultant spectra. A comparison of these "calculated" effects with the "actual" effects as computed with the actual spectra shows excellent agreement.

Splicing of tRNA precursors in Saccharomyces cerevisiae extracts proceeds in two steps; excision of the intervening sequence and ligation of the tRNA halves. The ability to resolve these two steps and the distinct physical properties of the endonuclease and ligase suggested that the splicing steps may not be concerted and that these two enzymes may act independently in vivo. A ligase competition assay was developed to examine whether the excision and ligation steps in tRNA splicing in vitro are concerted or independent. The ability of either yeast ligase or T4 ligase plus kinase to join the tRNA halves produced by endonuclease and the distinct structures of the reaction products provided the basis for the competition assay. In control reactions, joining of isolated tRNA halves formed by preincubation with endonuclease was measured. The ratio of yeast to T4 reaction products in these control assays reflected the ratio of the enzyme activities, as would be expected if each has equal access to the substrate. In splicing competition assays, endonuclease and pre-tRNA were added to ligase mixtures, and joining of the halves that were formed was measured. In these assays the products were predominantly those of the yeast ligase even when the T4 enzymes were present in excess. These results demonstrate preferential access of yeast ligase to the endonuclease products and provide evidence for the assembly of a functional tRNA splicing complex in vitro. This observation has important implications for the organization of the splicing components and of the gene expression pathway in vivo.

We have characterized the mechanism of action of a wheat germ RNA ligase which has been partially purified on the basis of its ability to participate in in vitro splicing of yeast tRNA precursors (Gegenheimer, P., Gabius, H-J., Peebles, C.L., and Abelson, J. (1983) J. Biol. Chem. 258, 8365-8373). The preparation catalyzes the ligation of oligoribonucleotide substrates forming a 2'-phosphomonoester, 3',5'-phosphodiester linkage. The 5' terminus of an RNA substrate can have either a 5'-hydroxyl or a 5'-phosphate. The 5'-phosphate, which for a 5'-hydroxyl substrate can be introduced by a polynucleotide kinase activity in the preparation, is incorporated into the ligated junction. The 3' terminus can have either a 2',3'-cyclic phosphate or a 2'-phosphate. 2',3'-Cyclic phosphates can be converted into 2'-phosphates by a 2',3'-cyclic phosphate, 3'-phosphodiesterase activity in the preparation. The 2'-phosphate of the ligated product is derived from the phosphate at the 3' terminus of the substrate. Ligation proceeds with the adenylylation of the 5'-phosphorylated terminus to form an intermediate with a 5',5'-phosphoanhydride bond.

Predicted single-stranded structure at the 3' splice site is a conserved feature among intervening sequences (IVSs) in eukaryotic nuclear tRNA precursors. The role of 3' splice site structure in splicing was examined through hexanucleotide insertions at a central intron position in the Saccharomyces cerevisiae tRNA gene. These insertions were designed to alter the structure at the splice site without changing its sequence. Endonuclease cleavage of pre-tRNA substrates was then measured in vitro, and suppressor activity was examined in vivo. A precursor with fully double-stranded structure at the 3' splice site was not cleaved by endonuclease. The introduction of one unpaired nucleotide at the 3' splice site was sufficient to restore cleavage, although at a reduced rate. We have also observed that guanosine at the antepenultimate position provides a second consensus feature among IVSs in tRNA precursors. Point mutations at this position were found to affect splicing although there was no specific requirement for guanosine. These and previous results suggest that elements of secondary and/or tertiary structure at the 3' end of IVSs are primary determinants in pre-tRNA splice site utilization whereas specific sequence requirements are limited.