The products of the HIRI and HIR2 genes have been defined genetically as repressors of histone gene transcription in S. cerevisiae. A mutation in either gene affects cell cycle regulation of three of the four histone gene loci; transcription of these loci occurs throughout the cell cycle and is no longer repressed in response to the inhibition of DNA replication. The same mutations also eliminate autogenous regulation of the HTA1-HTB1 locus by histones H2A and H2B. The HIR1 and HIR2 genes have been isolated, and their roles in the transcriptional regulation of the HTA1-HTB1 locus have been characterized. Neither gene encodes an essential protein, and null alleles derepress HTA1-HTB1 transcription. Both HIR genes are expressed constitutively under conditions that lead to repression or derepression of the HTAI gene, and neither gene regulates the expression of the other. The sequence of the HIR1 gene predicts an 88-kDa protein with three repeats of a motif found in the G. subunit of retinal transducin and in a yeast transcriptional repressor, Tupl. The sequence of the HIR2 gene predicts a protein of 98 kDa. Both gene products contain nuclear targeting signals, and the Hir2 protein is localized in the nucleus.
Mutations in the GSF2 gene cause glucose starvation phenotypes in Saccharomyces cerevisiae. We have isolated the HXT1 gene, which encodes a low-affinity, highcapacity glucose transporter, as a multicopy suppressor of a gsf2 mutation. We show that gsf2 mutants accumulate Hxt1p in the endoplasmic reticulum (ER) and that Gsf2p is a 46-kDa integral membrane protein localized to the ER. gsf2 mutants also display a galactose growth defect and abnormal localization of the galactose transporter Gal2p but are not defective in function or localization of the high-affinity glucose transporter Hxt2p. These findings suggest that Gsf2p functions in the ER to promote the secretion of certain hexose transporters.The HXT genes of the yeast Saccharomyces cerevisiae encode a family of hexose transporters with diverse kinetic properties and patterns of expression (1-4). Transcription of the HXT genes is regulated by the concentration of glucose in the environment, and this transcriptional regulation is a principal mechanism whereby cells adapt to fluctuations in glucose availability. For example, when glucose is abundant, genes encoding low-affinity glucose transporters are induced while genes encoding high-affinity glucose transporters are repressed. When glucose is scarce, this pattern of HXT gene transcription is reversed such that high-affinity glucose transporters are expressed (5). The HXT1 gene encodes a major low-affinity, high-capacity glucose transporter (K m ϭ100 mM) that is strongly induced in response to high levels of glucose (5-8). In this study, we have isolated HXT1 as a multicopy suppressor of mutations in GSF2 (glucose signaling factor).The GSF2 gene was previously identified in a screen for mutants defective in signaling the presence of high glucose levels (9). GSF2 encodes a protein with a putative transmembrane domain and a C-terminal dilysine motif for retrieval of transmembrane proteins to the ER (see Fig. 2 A; see also refs.9-11). In addition to relieving glucose repression of SUC2 and GAL10 transcription, gsf2 mutations cause a synthetic lethal phenotype in combination with snf1⌬, suggesting a functional relationship between GSF2 and SNF1. SNF1 encodes a protein-serine͞threonine kinase that is required to relieve transcriptional repression of many genes in response to glucose depletion (12).Here, we have isolated multicopy suppressors of the synthetic lethal phenotype of gsf2 and snf1 mutations. The recovery of HXT1 as a multicopy suppressor suggested a role for Gsf2p in glucose transporter function. We show that GSF2 encodes a 46-kDa integral membrane protein localized to the ER and that mutations in GSF2 lead to an accumulation of Hxt1p in the ER. These findings explain the glucose starvation and synthetic lethal phenotypes of gsf2 mutants. gsf2 mutations also affect the secretion of the galactose transporter Gal2p, but not the high-affinity glucose transporter Hxt2p. We suggest that Gsf2p functions in the ER to promote the secretion of certain hexose transporters. MATERIALS AND METHODSStrains, M...
The ability of the zeste moiety of l-galactosidase-zeste fusion proteins synthesized in Escherchia coli to bind specific DNA sequences was examined. Such fusion proteins recognize a region of the white locus upstream of the start of transcription; this region has previously been shown to be required for genetic interaction between the zeste and white loci. Another strong binding site was localized to a region between 50 and 205 nucleotides before the start of the Ubx transcriptional unit; expression of the bithorax complex is also known to be influenced by the zeste locus. Weaker binding sites were also seen in the vicinity of the bxd and Sgs4 genes, but it is currently unclear whether these binding sites play a role in transvection effects. The DNA-binding activity of the zeste protein is restricted to a domain of approximately 90 amino acids near the N terminus. This domain does not appear to contain homeobox or zinc finger motifs found in other DNA-binding proteins. The DNA-binding domain is not disrupted by any currently characterized zeste mutations.
The products of the HIR1 and HIR2 genes have been defined genetically as repressors of histone gene transcription in S. cerevisiae. A mutation in either gene affects cell cycle regulation of three of the four histone gene loci; transcription of these loci occurs throughout the cell cycle and is no longer repressed in response to the inhibition of DNA replication. The same mutations also eliminate autogenous regulation of the HTA1-HTB1 locus by histones H2A and H2B. The HIR1 and HIR2 genes have been isolated, and their roles in the transcriptional regulation of the HTA1-HTB1 locus have been characterized. Neither gene encodes an essential protein, and null alleles derepress HTA1-HTB1 transcription. Both HIR genes are expressed constitutively under conditions that lead to repression or derepression of the HTA1 gene, and neither gene regulates the expression of the other. The sequence of the HIR1 gene predicts an 88-kDa protein with three repeats of a motif found in the G beta subunit of retinal transducin and in a yeast transcriptional repressor, Tup1. The sequence of the HIR2 gene predicts a protein of 98 kDa. Both gene products contain nuclear targeting signals, and the Hir2 protein is localized in the nucleus.
The zeste locus plays a central role in transvection phenomena, where the synaptic pairing of chromosomes carrying genes with which zeste interacts influences the expression of these genes. To explore the possible functions of the zeste gene product in this process, we have determined the DNA sequences both of a fragment of Drosophila genomic DNA capable of rescuing mutant zeste phenotypes, and of a near full-length cDNA clone derived from the 2.4-kb zeste mRNA. These data show that the zeste gene is interrupted by two small introns, and suggest that the majority of zeste sequences are contained within an intron of another transcriptional unit of opposite polarity. A large region of the predicted zeste product is comprised almost exclusively of glutamine and alanine residues. A domain near the N terminus of this protein, which is sufficient for site-specific DNA binding, is highly charged, as is the C-terminal region of the protein. A breakpoint of the rearrangement In (1)e(bx), which is associated with a za-like phenotype, is found within sequences encoding the zeste product, and would produce a truncated protein. The neomorphic mutation zv77h is correlated with a 300-bp deletion of sequences determining the untranslated 5' leader of the zeste messenger, but may also remove the initiating ATG codon, resulting in a zeste protein with an altered N terminus.
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