Isolation and characterization of temperature-sensitive RNA polymerase II mutants of Saccharomyces cerevisiae.

Three independent, recessive, temperature-sensitive (Ts-) conditional lethal mutations in the largest subunit of Saccharomyces cerevisiae RNA polymerase II (RNAP II) have been isolated after replacement of a portion of the wild-type gene (RPO21) by a mutagenized fragment of the cloned gene. Measurements of cell growth, viability, and total RNA and protein synthesis showed that rpo21-1, rpo21-2, and rpo21-3 mutations caused a slow shutoff of RNAP II activity in cells shifted to the nonpermissive temperature (39 degrees C). Each mutant displayed a distinct phenotype, and one of the mutant enzymes (rpo21-1) was completely deficient in RNAP II activity in vitro. RNAP I and RNAP III in vitro activities were not affected. These results were consistent with the notion that the genetic lesions affect RNAP II assembly or holoenzyme stability. DNA sequencing revealed that in each case the mutations involved nonconservative amino acid substitutions, resulting in charge changes. The lesions harbored by all three rpo21 Ts- alleles lie in DNA sequence domains that are highly conserved among genes that encode the largest subunits of RNAP from a variety of eucaryotes; one mutation lies in a possible Zn2+ binding domain.

Isolation of the SUP45 omnipotent suppressor gene of Saccharomyces cerevisiae and characterization of its gene product.

The Saccharomyces cerevisiae SUP45+ gene has been isolated from a genomic clone library by genetic complementation of paromomycin sensitivity, which is a property of a mutant strain carrying the sup45-2 allele. This plasmid complements all phenotypes associated with the sup45-2 mutation, including nonsense suppression, temperature sensitivity, osmotic sensitivity, and paromomycin sensitivity. Genetic mapping with a URA3+-marked derivative of the complementing plasmid that was integrated into the chromosome by homologous recombination demonstrated that the complementing fragment contained the SUP45+ gene and not an unlinked suppressor. The SUP45+ gene is present as a single copy in the haploid genome and is essential for viability. In vitro translation of the hybrid-selected SUP45+ transcript yielded a protein of Mr = 54,000, which is larger than any known ribosomal protein. RNA blot hybridization analysis showed that the steady-state level of the SUP45+ transcript is less than 10% of that for ribosomal protein L3 or rp59 transcripts. When yeast cells are subjected to a mild heat shock, the synthesis rate of the SUP45+ transcript was transiently reduced, approximately in parallel with ribosomal protein transcripts. Our data suggest that the SUP45+ gene does not encode a ribosomal protein. We speculate that it codes for a translation-related function whose precise nature is not yet known.

Molecular cloning and biosynthetic regulation of cry1 gene of Saccharomyces cerevisiae.

The cryptopleurine resistance gene, cry1, of Saccharomyces cerevisiae has been molecularly cloned using genetic complementation of cryptopleurine sensitivity by the cryptopleurine resistance gene contained in a clone library prepared from DNA of a cryptopleurine resistant strain. Analysis of RNA transcripts indicated that the cry1 gene is the template for a transcript of approximately 900 bases and that the primary transcript contains an intron of approximately 300 bases. In vitro hybrid selection translation experiments indicated that this transcript encodes a protein of molecular weight 17 kilodaltons which on two-dimensional SDS polyacrylamide gels exactly coincides with ribosomal protein rp59. Further analysis showed that when the gene was present on a plasmid of about five copies per cell the amount of messenger RNA was elevated approximately five-fold compared to a cell that had only a single chromosomal copy. The rate of synthesis of ribosomal protein rp59 was not detectably elevated. These data suggest that the cry1 gene is regulated, at least in part, post-transcriptionally.

Identification, molecular cloning, and mutagenesis of Saccharomyces cerevisiae RNA polymerase genes.

Three different regions of Saccharomyces cerevisiae DNA were identified by using as hybridization probe a fragment of Drosophila melanogaster DNA that encodes an RNA polymerase II (EC 2.7.7.6) polypeptide. Two of these regions have been molecularly cloned. Each contains a sequence related not only to the D. melanogaster DNA fragment that was used as a probe in its isolation but also to the immediately adjacent DNA fragment of the D. melanogaster RNA polymerase II gene. The two cloned S. cerevisiae DNA sequences are each the template for single transcripts in vivo, one of 5.9 kilobases and the other of 4.6 kilobases. In vitro translation of hybrid-selected cellular RNA indicated that the former locus encodes a protein of Mr 220,000, equal in size to the largest polypeptide subunit of S. cerevisiae RNA polymerase II. Disruption of either gene by targeted integration of URA3+ DNA demonstrated that each is single-copy and essential in a haploid genome. We suggest that these S. cerevisiae loci are members of a family of related genes encoding the largest subunit polypeptides of RNA polymerases I, II, and III.

GAL11P: a yeast mutation that potentiates the effect of weak GAL4-derived activators.

A mutant yeast in which a weak GAL4-derived activator functions as a strong activator bears a single mis-sense mutation in GAL11 (a.k.a. SPT13). The first 74 amino acids of GAL4, including the zinc-dependent DNA binding region, attached to an acidic activating sequence, are sufficient to respond both to GAL11 and to our mutant GAL11P (potentiator). PPR1, a yeast activator with a similar zinc finger sequence, also responds to GAL11 and to GAL11P, whereas regulators bearing unrelated DNA binding motifs do not. GAL11 itself works as a strong activator when tethered to DNA by fusion to the bacterial LexA protein, and deletion of GAL11 is known to cause a 5- to 10-fold reduction in GAL4 activity. We suggest that a complex of GAL4 and GAL11 constitutes a particularly strong activator; evidence that the putative GAL4-GAL11 complex ordinarily forms preferentially on DNA suggests a biological rationale for GAL11 action.