We have determined that TPD3, a gene previously identified in a screen for mutants defective in tRNA biosynthesis, most likely encodes the A regulatory subunit of the major protein phosphatase 2A species in the yeast Saccharomyces cerevisiae. The predicted amino acid sequence of the product of TPD3 is highly homologous to the sequence of the mammalian A subunit of protein phosphatase 2A. In addition, antibodies raised against Tpd3p specifically precipitate a significant fraction of the protein phosphatase 2A activity in the cell, and extracts of tpd3 strains yield a different chromatographic profile of protein phosphatase 2A than do extracts of isogenic TPD3 strains. tpd3 deletion strains generally grow poorly and have at least two distinct phenotypes. At reduced temperatures, tpd3 strains appear to be defective in cytokinesis, since most cells become multibudded and multinucleate following a shift to 13°C. This is similar to the phenotype obtained by overexpression of the protein phosphatase 2A catalytic subunit or by loss of CDCSS, a gene that encodes a protein with homology to a second regulatory subunit of protein phosphatase 2A. At elevated temperatures, tpd3 strains are defective in transcription by RNA polymerase III. Consistent with this in vivo phenotype, extracts of tpd3 strains fail to support in vitro transcription of tRNA genes, a defect that can be reversed by addition of either purified RNA polymerase III or TFIUIB. These results reinforce the notion that protein phosphatase 2A affects a variety of biological processes in the cell and provide an initial identification of critical substrates for this phosphatase.

In Saccharomyces cerevisiae, expression of the ADH2 gene is undetectable during growth on glucose. The transcription factor ADR1 is required to fuly activate expression when glucose becomes depleted. Partial activation during growth on

Unlike the majority of genes encoding small nuclear RNAs, which are transcribed by RNA polymerase B, the U6 gene contains features found in both class B and class C genes, indicating the involvement of a combination of transcription factors normally specific to each class of genes. We present direct genetic and biochemical evidence that the U6 gene of Saccharomyces cerevisiae is transcribed by RNA polymerase C in vivo as well as in vitro. A mutant strain with a temperature‐sensitive defect in the large subunit of RNA polymerase C that results in defective transcription of tRNA and 5S RNA genes shows a corresponding defect in U6 RNA levels. Also, purified RNA polymerase C transcribes the U6 gene when supplemented with partially purified TFIIIB. The other class C transcription factors, TFIIIA and Tau (TFIIIC), are not required in this system.

Yeast transcription factor tau forms a stable complex with tRNA genes. Using this property, the factor could be highly purified on a specific tDNA column. The purified factor was found by DNA footprinting to protect the whole yeast tRNA3Glu gene from position ‐8 to +81. A DNase‐sensitive site was retained in the middle of the gene on both strands. The 3′ border of the complex was mapped by exonuclease digestion at +88, just downstream of the termination signal. The 5′ limit of the complex was found at position ‐11. However, upon prolonged incubation with exonuclease, the ‐11 blockage disappeared and the DNA molecules were digested to position +30 to 38 in the middle of the gene. Contact points at guanine residues were identified by dimethyl sulphate protection experiments. Reduced methylation of G residues in the presence of factor was found solely within the A block and in the B block region. All six invariant GC pairs (i.e., G10, G18, G19 and G53, C56 and C61) were found to have strong contacts with the factor. These results show that tau factor interacts with both the 5′ and 3′ half of the tRNA3Glu gene, with the B block region being the predominant binding site. The presence of this dual binding site suggests a model in which the factor would bind alternately at the A and B block regions to allow transcription of the internal promoter by RNA polymerase C.

A yeast extract was fractionated to resolve the factors involved in the transcription of yeast tRNA genes. An in vitro transcription system was reconstituted with two separate protein fractions and purified RNA polymerase C (III). Optimal conditions for tRNA synthesis have been determined. One essential component, termed tau factor, was partially purified by conventional chromatographic methods on heparin‐agarose and DEAE‐Sephadex; it sedimented as a large macromolecule in glycerol gradients (mol. wt. approximately 300 000). tau factor was found to form a stable complex with the tRNA gene in the absence of other transcriptional components. Complex formation is very fast, is not temperature dependent between 10 degrees C and 25 degrees C and does not require divalent cations. The factor‐DNA complex is stable for at least 30 min at high salt concentration (0.1 M ammonium sulfate). These results indicate that gene recognition by a specific factor is a primary event in tRNA synthesis.

Yeast transcription factor 7 (transcription factor IRIC) specifically interacts with tRNA genes, binding to both the A block and the B block elements of the internal promoter. To study the influence of A block-B block spacing, we analyzed the binding of purified X protein to a series of internally deleted yeast tRNAje genes with A and B blocks separated by 0 to 74 base pairs. Optimal binding occurred with genes having A block-B block distances of30-60 base pairs; the relative helical orientation of the A and B blocks was unimportant. Results from DNase I "footprinting" and A exonuclease protection experiments were consistent with these findings and further revealed that changes in A block-B block distance primarily affect the ability of 7to interact with A block sequences; B block interactions are unaltered. When the A block-B block distance is 17 base pairs or less, 7 interacts with a sequence located 15 base pairs upstream of the normal A block, and a new RNA initiation site is observed by in vitro transcription. We propose that the initial binding of 7 to the B block activates transcription by enhancing its ability to bind at the A block, and that the A block interaction ultimately directs initiation by RNA polymerase HI.The genes for eukaryotic cytoplasmic tRNAs (tDNAs) as well as 7S RNA, 4.5S RNA, and related pseudogenes all possess an intragenic split promoter that directs specific initiation by RNA polymerase III (pol III) (1). Transcription of these genes also requires accessory factors IIIB and IIIC (2, 3). The yeast transcription factor IIIC, given the name r (4, 5), is a macromolecule that binds to sequences within the transcribed regions of these genes (6

Transcription of eukaryotic transfer RNA genes involves, as a primary event, the stable binding of a protein factor to the intragenic promoter. The internal control region is composed of two non-contiguous conserved sequence elements, the A and B blocks. These are variably spaced depending on the genes. tau, a large transcription factor purified from yeast cells, interacts with these two control elements as shown by DNase I footprinting, exonuclease digestion, dimethyl sulphate protection experiments and by analysis of point mutations. Here we used a limited proteolysis treatment to obtain a smaller form of tau with drastically altered DNA binding properties. A protease-resistant domain interacts solely with the B block region of tRNA genes.

The role of the Acanthamoeba casteUlanii TATA-binding protein (TBP) in transcription was examined. Specific antibodies against the nonconserved N-terminal domain of TBP were used to verify the presence of TBP in the fundamental transcription initiation factor for RNA polymerase I, TIF-IB, and to demonstrate that TBP is part of the committed initiation complex on the rRNA promoter. The same antibodies inhibit transcription in all three polymerase systems, but they do so differentially. Oligonucleotide competitors were used to evaluate the accessibility of the TATA-binding site in TIF-IB, TFIID, and TFIIIB. The results suggest that insertion of TBP into the polymerase II and III factors is more similar than insertion into the polymerase I factor.While the transcription systems of eukaryotic RNA polymerases I, II, and III obviously share some characteristics, initiation mechanisms for these transcription systems have been largely studied separately. The polymerases themselves have five subunits in common, seven in the case of the "odd pols," RNA polymerases I and III (45). Nevertheless, the general transcription factors involved with each polymerase have been examined in isolation, perhaps masking important generalizations about their functions. This approach began to change when it was discovered that some of the genes for small nuclear RNAs (snRNAs) are transcribed by polymerase III whereas most are transcribed by polymerase II (27,32,35,47 The full impact of this factor overlap was perhaps not realized because polymerase I still appeared to use dedicated factors. However, a flurry of studies (6,7,44,56) revealed that TBP was required for transcription of all genes. TBP is now known to be a subunit of TFIID (41), TFIIIB (17,24,48,55), and human TIF-IB (SL1) (6). TBP is associated with additional subunits (TAFs) to make up the functional factors (reviewed in reference 41). The TAFs appear to be different for each polymerase system (reviewed in reference 42), although overlap of TAFs has not been rigorously ruled out. All the TBP-containing factors are pivotal for their respective polymerases. Indeed, TFIIIB and TIF-IB are fundamental transcription factors; i.e., they appear to be responsible for the repetitive recruitment of RNA polymerase during successive rounds of initiation (16; reviewed in references 36 and 37).The manner by which the TBP-containing factor is recruited to the promoter differs from gene to gene. For RNA polymerase III genes with type I (SS RNA) or type II (tRNA, VAI, Alu, EBER, 7SL, 4.5S) internal control regions, additional general transcription factors are required to assemble TFIIIB onto the promoter (reviewed in references 10-12). Initiation complex formation on 5S and tRNA genes is an organized process in which the factors bind in an obligatory order, each relying on protein-DNA and protein-protein interactions with a previously bound factor(s) to join the complex. On some genes the DNA interaction site for TFIIIB is sequence specific, whereas on others specific sequence recognition is no...