A yeast transcription system for the 5S rRNA gene

Cross talk between tRNA and rRNA synthesis in Saccharomyces cerevisiae

Temperature-sensitive RNA polymerase III (rpc160-112 and rpc160-270) mutants were analyzed for the synthesis of tRNAs and rRNAs in vivo, using a double-isotopic-labeling technique in which cells are pulse-labeled with [(33)P]orthophosphate and coextracted with [(3)H]uracil-labeled wild-type cells. Individual RNA species were monitored by Northern blot hybridization or amplified by reverse transcription. These mutants impaired the synthesis of RNA polymerase III transcripts with little or no influence on mRNA synthesis but also largely turned off the formation of the 25S, 18S, and 5.8S mature rRNA species derived from the common 35S transcript produced by RNA polymerase I. In the rpc160-270 mutant, this parallel inhibition of tRNA and rRNA synthesis also occurred at the permissive temperature (25 degrees C) and correlated with an accumulation of 20S pre-rRNA. In the rpc160-112 mutant, inhibition of rRNA synthesis and the accumulation of 20S pre-rRNA were found only at 37 degrees C. The steady-state rRNA/tRNA ratio of these mutants reflected their tRNA and rRNA synthesis pattern: the rpc160-112 mutant had the threefold shortage in tRNA expected from its preferential defect in tRNA synthesis at 25 degrees C, whereas rpc160-270 cells completely adjusted their rRNA/tRNA ratio down to a wild-type level, consistent with the tight coupling of tRNA and rRNA synthesis in vivo. Finally, an RNA polymerase I (rpa190-2) mutant grown at the permissive temperature had an enhanced level of pre-tRNA, suggesting the existence of a physiological coupling between rRNA synthesis and pre-tRNA processing.

5S-rRNA genes in rice embryos

The 5S-rRNA from the ungerminated and 48-h-germinated rice embryos differs from the wheat, rye and maize by two nucleotides. The 48-h-germinated embryos contain another species of 5S-rRNA which differs by 3 nucleotides from the ungerminated embryos, thereby showing the expression of two 5S-rRNA genes during germination. The 5S-rRNA genes are present in tandem repeats of a 0.3-kb sequence with some length heterogeneity in the rice genome. The 5S-rRNA gene that was sequenced is identical to that of wheat and maize, except for two nucleotides, C and T, which are interchanged at positions 107 and 117. The insert of continuous 5S-rRNA gene in pBR322 was transcribed in vitro much more efficiently than the discontinuous gene. There was no homology between the 184-bp spacer sequence of 5S-rRNA genes in rice and other systems except the presence of the oligo(T) transcription terminator sequence.

Eukaryotic RNA polymerases

This review will attempt to cover the present information on the multiple forms of eukaryotic DNA-dependent RNA polymerases, both at the structural and functional level. Nuclear RNA polymerases constitute a group of three large multimeric enzymes, each with a different and complex subunit structure and distinct specificity. The review will include a detailed description of their molecular structure. The current approaches to elucidate subunit function via chemical modification, phosphorylation, enzyme reconstitution, immunological studies, and mutant analysis will be described. In vitro reconstituted systems are available for the accurate transcription of cloned genes coding for rRNA, tRNA, 5 SRNA, and mRNA. These systems will be described with special attention to the cellular factors required for specific transcription. A section on future prospects will address questions concerning the significance of the complex subunit structure of the nuclear enzymes; the organization and regulation of the gene coding for RNA polymerase subunits; the obtention of mutants affected at the level of factors, or RNA polymerases; the mechanism of template recognition by factors and RNA polymerase.

Structure and transcription of eukaryotic tRNA genes

The availability of cloned tRNA genes and a variety of eukaryotic in vitro transcription systems allowed rapid progress during the past few years in the characterization of signals in the DNA-controlling gene transcription and in the processing of the precurser RNAs formed. This will be the subject matter discussed in this review.

The duck alpha A globin but not the yeast actin gene is transcribed by a HeLa cell extract

We have investigated the transcription in a HeLa whole-cell extract of two evolutionary widely separated structural genes coding for duck alpha A globin and yeast actin. Transcription of isolated DNA fragments of the duck alpha A globin gene increases linearly up to relatively high concentrations of DNA. Size analyses and S1 mapping of the transcripts synthesized in vitro on either linear DNA fragments or supercoiled templates reveal that the alpha A globin RNA is initiated at the in vivo cap site and remains unspliced. The same assay conditions were used to transcribe the yeast actin gene. In contrast to the duck gene, size analyses and S1 mapping of the RNA products synthesized on both linear DNA fragments and the supercoiled template containing the actin gene show that the transcripts found in vitro do not stem from the in vivo cap site. The promoter of the yeast actin gene is not recognized in this system in vitro.

The in vivo and in vitro initiation site for transcription of the rRNA operon of Saccharomyces carlsbergensis

We have performed a detailed analysis of the transcription initiation of the rRNA operon in the yeast Saccharomyces carlsbergensis. Electron microscopic analysis of R-looped pre-rRNA molecules together with a very sensitive S1-nuclease mapping showed the use of only a single transcription start at about 700 bp upstream of the 17S rRNA gene and not of the minor start sites proposed for the very closely related species S. cerevisiae by others [Bayev et al. (5), Swanson and Holland (6)]. The sequence of 730 bp of the initiating region is presented. In vitro transcription in concentrated lysates of yeast spheroplasts in the presence of (gamma-SH)ATP or (gamma-SH)GTP, followed by purification of the in vitro initiated RNA via Hg-agarose, revealed that on the endogenous template exactly the same site is used for transcription initiation as in vivo.

Isolation of a class C transcription factor which forms a stable complex with tRNA genes

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.