Mutation in the prp12+ gene encoding a homolog of SAP130/SF3b130 causes differential inhibition of pre-mRNA splicing and arrest of cell-cycle progression in Schizosaccharomyces pombe

prp12-1 is one of the mutants defective in pre-mRNA splicing at a nonpermissive temperature in Schizosaccharomyces pombe. We found that the prp12+ gene encodes a protein highly homologous with a human splicing factor, SAP130/SF3b130, a subunit of a U2 snRNP-associated complex SF3b. Prp12p was shown to interact genetically with Prp10p that is a homolog of SAP155/SF3b155, another subunit in SF3b, suggesting that Prp12p is a functional homolog of human SAP130/SF3b130. Prp12p tagged with GFP is uniformly localized in the nuclear DNA region. In addition to pre-mRNA splicing defects, the prp12-1 mutant produced elongated cells, a typical phenotype of cell division cycle (cdc) mutants, suggesting a possible link between pre-mRNA splicing and cell-cycle progression. We examined kinetics of splicing defects in prp12-1 and several other prp mutants using northern blot hybridization and found that, among all the tested pre-mRNAs, only Tflld pre-mRNA with low splicing efficiency showed detectable splicing defects at the nonpermissive temperature in prp12-1. In addition, we found that other prp mutants with the cdc phenotype also showed differential splicing defects in tested pre-mRNAs at the nonpermissive temperature. On the other hand, prp mutants that do not exhibit the cdc phenotype showed a rapid and complete block of pre-mRNA splicing in all the tested pre-mRNAs at the nonpermissive temperature, indicating that prp mutants with weak splicing defects have a tendency to exhibit the cdc phenotype. These results suggest that the cdc phenotype in prp12-1 is caused by a selective reduction of spliced transcripts encoding a protein (or proteins) required for G2/M transition.

Characterization of the ptr6(+) gene in fission yeast: a possible involvement of a transcriptional coactivator TAF in nucleocytoplasmic transport of mRNA

Transport of mRNA from the nucleus to the cytoplasm is one of the important steps in gene expression in eukaryotic cells. To elucidate a mechanism of mRNA export, we identified a novel ptr [poly(A)+ RNA transport] mutation, ptr6, which causes accumulation of mRNA in the nucleus and inhibition of growth at the nonpermissive temperature. The ptr6(+) gene was found to encode an essential protein of 393 amino acids, which shares significant homology in amino acid sequence with yTAFII67 of budding yeast Saccharomyces cerevisiae and human hTAFII55, a subunit of the general transcription factor complex TFIID. A Ptr6p-GFP fusion protein is localized in the nucleus, suggesting that Ptr6p functions there. Northern blot analysis using probes for 10 distinct mRNAs showed that the amount of tbp+ mRNA encoding the TATA-binding protein is increased five- to sixfold, whereas amounts of others are rapidly decreased at the nonpermissive temperature in ptr6-1. ptr6 has no defects in nuclear import of an NLS-GFP fusion protein. These results suggest that Ptr6p required for mRNA transport is a Schizosaccharomyces pombe homologue of yTAFII67 and hTAFII55. This is the first report suggesting that a TAF is involved in the nucleocytoplasmic transport of mRNA in addition to the transcription of the protein-coding genes.

The fission yeast prp10(+) gene involved in pre-mRNA splicing encodes a homologue of highly conserved splicing factor, SAP155

In the fission yeast Schizosaccharomyces pombe, 14 prp (pre-mRNAprocessing) mutants have been isolated to date. We cloned the prp10(+) gene by complementation of the temperature-sensitive growth of prp10. Five types of transcripts were found that were alternatively spliced with respect to two possible introns located in the 5'-terminal region. Three of them are probably functional and code for putative proteins of approximately 1200 amino acids. Proteins highly homologous to Prp10p are present in other organisms, one of which is a human spliceosome-associated protein SAP155, a subunit of the splicing factor complex SF3. The C-terminal two-thirds of Prp10p is highly conserved among species, and contains consensus repeats for the regulatory subunit A of protein phosphatase PP2A. A gene disruption experiment indicated that the prp10(+) gene is essential for viability in S.pombe. Prp10p tagged with GFP is predominantly localized in the nuclear DNA region. A series of deletions showed that the less conserved N-terminal region of approximately 300 amino acids in Prp10p is dispensable, although the corresponding region was thought to play important roles in the mammalian splicing system.

The prp1+ gene required for pre-mRNA splicing in Schizosaccharomyces pombe encodes a protein that contains TPR motifs and is similar to Prp6p of budding yeast

The prp (pre-mRNA processing) mutants of the fission yeast Schizosaccharomyces pombe have a defect in pre-mRNA splicing and accumulate mRNA precursors at a restrictive temperature. One of the prp mutants, prp1-4, also has a defect in poly(A)+ RNA transport. The prp1+ gene encodes a protein of 906 amino acid residues that contains 19 repeats of 34 amino acids termed tetratrico peptide repeat (TPR) motifs, which were proposed to mediate protein-protein interactions. The amino acid sequence of Prp1p shares 29.6% identity and 50.6% similarity with that of the PRP6 protein of Saccharomyces cerevisiae, which is a component of the U4/U6 snRNP required for spliceosome assembly. No functional complementation was observed between S. pombe prp1+ and S. cerevisiae PRP6. We examined synthetic lethality of prp1-4 with the other known prp mutations in S. pombe. The results suggest that Prp1p interacts either physically or functionally with Prp4p, Prp6p and Prp13p. Interestingly, the prp1+ gene was found to be identical with the zer1+ gene that functions in cell cycle control. These results suggest that Prp1p/Zer1p is either directly or indirectly involved in cell cycle progression and/or poly(A)+ RNA nuclear export, in addition to pre-mRNA splicing.

Isolation and molecular characterization of mRNA transport mutants in Schizosaccharomyces pombe

Nucleocytoplasmic transport of mRNA is essential for eukaryotic gene expression. However, how mRNA is exported from the nucleus is mostly unknown. To elucidate the mechanisms of mRNA transport, we took a genetic approach to identify genes, the products of which play a role in that process. From about 1000 temperature -sensitive (ts- or cs-) mutants, we identified five ts- mutants that are defective in poly(A)+ RNA transport by using a situ hybridization with an oligo(dT)50 as a probe. These mutants accumulate poly(A)+ RNA in the nuclei when shifted to a nonpermissive temperature. All five mutations are tightly linked to the ts- growth defects, are recessive, and fall into four different groups designated as ptr 1-4 (poly(A)+ RNA transport). Interestingly, each group of mutants has a differential localization pattern of poly(A)+ RNA in the nuclei at the nonpermissive temperature, suggesting that they have defects at different steps of the mRNA transport pathway. Localization of a nucleoplasmin-green fluorescent protein fusion suggests that ptr2 and ptr3 have defects also in nuclear protein import. Among the isolated mutants, only ptr2 showed a defect in pre-mRNA splicing. We cloned the ptr2+ and ptr3+ genes and found that they encode Schizosaccharomyces pombe homologues of the mammalian RCC1, a guanine nucleotide exchange factor for RAN/TC4, and the ubiquitin-activating enzyme E1 involved in ubiquitin conjugation, respectively. The ptr3+ gene is essential for cell viability, and Ptr3p tagged with green fluorescent protein was localized in both the nucleus and the cytoplasm. This is the first report suggesting that the ubiquitin system plays a role in mRNA export.

Isolation of novel pre-mRNA splicing mutants of Schizosaccharomyces pombe

New prp (pre-mRNA processing) mutants of the fission yeast Schizosaccharomyces pombe were isolated from a bank of 700 mutants that were either temperature sensitive (ts-) or cold sensitive (cs-) for growth. The bank was screened by Northern blot analysis with probes complementary to S. pombe U6 small nuclear RNA (sn RNA), the gene for which has a splicesomal (mRNA-type) intron. We identified 12 prp mutants that accumulated the U6 snRNA precursor at the nonpermissive temperature. All such mutants were also found to have defects in an early step of TFIID pre-mRNA splicing at the nonpermissive temperature. Complementation analyses showed that seven of the mutants belong to six new complementation groups designated as prp8 and prp10-prp14, whereas the five other mutants were classified into the known complementation groups prp1, prp2 and prp3. Interestingly, some of the isolated prp mutants produced elongated cells at the nonpermissive temperature, which is a phenotype typical of cell division cycle (cdc) mutants. Based on these findings, we propose that some of the wild-type products from these prp+ genes play important roles in the cellular processes of pre-mRNA splicing and cell cycle progression.

A connection between pre-mRNA splicing and the cell cycle in fission yeast: cdc28+ is allelic with prp8+ and encodes an RNA-dependent ATPase/helicase

The fission-yeast gene cdc28+ was originally identified in a screen for temperature-sensitive mutants that exhibit a cell-division cycle arrest and was found to be required for mitosis. We undertook a study of this gene to understand more fully the general requirements for entry into mitosis. Cells carrying the conditional lethal cdc28-P8 mutation divide once and arrest in G2 after being shifted to the restrictive temperature. We cloned the cdc28+ gene by complementation of the temperature-sensitive growth arrest in cdc28-P8. DNA sequence analysis indicated that cdc28+ encodes a member of the DEAH-box family of putative RNA-dependent ATPases or helicases. The Cdc28 protein is most similar to the Prp2, Prp16, and Prp22 proteins from budding yeast, which are required for the splicing of mRNA precursors. Consistent with this similarity, the cdc28-P8 mutant accumulates unspliced precursors at the restrictive temperature. Independently, we isolated a temperature-sensitive pre-mRNA splicing mutant prp8-1 that exhibits a cell-cycle phenotype identical to that of cdc28-P8. We have shown that cdc28 and prp8 are allelic. These results suggest a connection between pre-mRNA splicing and progression through the cell cycle.

An mRNA-type intron is present in the Rhodotorula hasegawae U2 small nuclear RNA gene

Splicing an mRNA precursor requires multiple factors involving five small nuclear RNA (snRNA) species called U1, U2, U4, U5, and U6. The presence of mRNA-type introns in the U6 snRNA genes of some yeasts led to the hypothesis that U6 snRNA may play a catalytic role in pre-mRNA splicing and that the U6 introns occurred through reverse splicing of an intron from an mRNA precursor into a catalytic site of U6 snRNA. We characterized the U2 snRNA gene of the yeast Rhodotorula hasegawae, which has four mRNA-type introns in the U6 snRNA gene, and found an mRNA-type intron of 60 bp. The intron of the U2 snRNA gene is present in the highly conserved region immediately downstream of the branch site recognition domain. Interestingly, we found that this region can form a novel base pairing with U6 snRNA. We discuss the possible implications of these findings for the mechanisms of intron acquisition and for the role of U2 snRNA in pre-mRNA splicing.

Application of encapsulated enzyme dispersed in a continuous stirred tank reactor

An application of encapsulated lipase to the hydrolysis of triacetin (triglyceride of acetic acid) was carried out with a continuous stirred tank reactor, in which the encapsulated enzyme was dispersed. An automatic control device to control pH of the reaction mixture at a desired level was designed and installed in the reactor system. Conversion of triacetin at the steady state operation with pH controlled became significantly higher than that without pH control. A particular kinetic model proposed by the authors, which regarded the mass-transfer through the wall of microcapsules as a dominant resistance to the overall reaction rate, was also applicable to simulate the behaviour of CSTR system as in the case of packed-bed reactor.

Application of encapsulated enzyme as a continuous packed-bed reactor

In the previous report, microencapsulation of lipase employing a (w/o)/w multiple phase emulsion technique, with 2:1 polystyrene (PS)-SBR mixture being used as a wall material, was proposed. Catalysis of the encapsulated enzyme was investigated, and the hydrolysis of triacetin (triglyceride of acetic acid) was successfully simulated by the reaction model based upon the Michaelis-Menten mechanism. Other factors affecting the mechanism such as the mass-transfer resistance of the substrate molecules through the wall and the decrease in pH due to the formation of acetic acid were also taken into consideration. In this report, the particular microcapsules were applied to the continuous tubular reactor system, essentially a packed column reactor, and longevity and mechanical strength of the microcapsules were fully demonstrated. The reaction model derived for a well-stirred batch reactor was also applicable to simulate the behaviour in the packed-column reactor as it was proved that there is no mass transfer resistance between the reactant stream and the surface of microcapsules. The observed data agreed quite well with the calculated values. Similarity of the behaviours of catalysis observed between two reactor systems was thoroughly confirmed. No leakage of the enzyme was detected after repeated usage over the duration of a few months, the temperature being maintained in the range between 293 and 323 K, and pH reset after each operation. Commercial feasibility of the microcapsules for the enzyme catalysis with substrates, small enough to permeate through the wall, was established by these fundamental investigations.

Immobilization of enzyme by microencapsulation and application of the encapsulated enzyme in the catalysis

Microencapsulation of lipase (Pseudomonas fluorescens) was carried out using (W/O)/W two-phase emulsion technique. Polystyrene (PS) and Styrene-Butadiene Rubber (SBR) were utilized as wall materials either separately or in mixture. A particular composition of 2:1 PS-SBR yielded homogeneous and tough wall structure, resilient to the impact and tight confinement of enzyme macromolecules. Performance of the encapsulated enzyme was evaluated employing the hydrolysis of triacetin (triglyceride of acetic acid) as a model substrate of the enzyme catalysis. A mathematical model was developed to simulate the behaviour of hydrolysis, which was derived under the assumption that the diffusion of small molecules (substrate and products) through the wall of microcapsules plays a dominant role to the reaction rate. Inhibition of the reaction by the decreasing pH due to the release of acetic acid was also taken into account. The calculated values agreed quite well with the observed data.