Ribosomal frameshifting in yeast viruses

Proper maintenance of translational reading frame by ribosomes is essential for cell growth and viability. In the last 10 years it has been shown that a number of viruses induce ribosomes to shift reading frame in order to regulate the expression of gene products having enzymatic functions. Studies on ribosomal frameshifting in viruses of yeast have been particularly enlightening. The roles of viral mRNA sequences and secondary structures have been elucidated and a picture of how these interact with host chromosomal gene products is beginning to emerge. The efficiency of ribosomal frameshifting is important for viral particle assembly, and has identified ribosomal frameshifting as a potential target for antiviral agents. The availability of mutants of host chromosomal gene products involved in maintaining the efficiency of ribosomal frameshifting bodes well for the use of yeast in future studies of ribosomal frameshifting.     

Schizosaccharomyces pombe RNase MRP RNA is homologous to metazoan RNase MRP RNAs and may provide clues to interrelationships between RNase MRP and RNase P

RNase MRP and RNase P ribonucleoproteins are structurally and functionally similar across a large evolutionary distance. To better characterize possible complex interrelationships between these two enzymes, we have employed the fission yeast Schizosaccharomyces pombe. Unlike Saccharomyces cerevisiae, S. pombe is believed to harbour only one genetic locus for the RNA component of RNase P and does not contain a known mitochondrially encoded RNase P RNA. We have identified the single nuclear gene for the RNA component of RNase MRP in S. pombe, mrp-1, by homology to vertebrate RNase MRP RNAs. The mrp-1 gene encodes an RNA of maximum mature length 400 nucleotides that shares a high degree of identity, in evolutionarily conserved regions, to both vertebrate RNase MRP RNAs and S. pombe RNase P RNA. Disruption of mrp-1 in the diploid strain SP826 and sporulation of tetrads resulted in a 2 dead:2 viable segregation, consistent with the gene being essential. Lethality is rescued by a plasmid-borne copy of mrp-1. Partially purified ribonucleoprotein RNase MRP activity correctly and efficiently processed all previously characterized heterologous mitochondrial RNA substrates. The compact mitochondrial genome of S. pombe contains sequence elements with >50% identity to mammalian D-loop CSBI and CSBII elements. The identification of mrp-1 in S. pombe should facilitate not only comparisons between the related ribonucleoproteins RNase MRP and RNase P, but should also provide an opportunity for genetic elucidation of RNase MRP function in a situation reflective of the animal kingdom.     

The complete sequence of a 7.5 kb region of chromosome III from Saccharomyces cerevisiae that lies between CRY1 and MAT.

We report the sequence of a 7.5 kb region lying between the CRY1 and MAT loci of chromosome III from Saccharomyces cerevisiae. This region lies in the overlap between two major contigs used for the generation of the complete nucleotide sequence of this chromosome. Comparison of this sequence with those reported previously for this overlap [Thierry et al. (1990) Yeast 6, 521; Jia et al. (1991) Yeast 7, 413] reveals 38 nucleotide differences, 45% of which generate changes in the amino acid sequences of the four genes in this region (YCR591, YCR592, YCR521 and YCR522). These differences appear to reflect true sequence polymorphisms between the two yeast strains used to generate the clones used in the sequencing project. Three of the four genes in this region display weak homologies to proteins in the PIR database. Some properties of YCR521 are analogous to those of ribosomal protein genes. However, the functions of all four genes remain obscure.     

The in situ architecture of Escherichia coli ribosomal RNA derived by electron spectroscopic imaging and three-dimensional reconstruction

The structures of the large and small ribosomal subunits of Escherichia coli were reconstructed using spectroscopic electron microscopy and quaternion-assisted angular reconstitution to resolutions of better than 4 nm. In addition, the distributions of phosphorus within these complexes were reconstructed. The three-dimensional reconstruction of the distribution of this atomic element is an extension of microanalysis (in two dimensions) for phosphorus identification and mapping, as a signature of the arrangement of the phosphate backbones of the constituent ribosomal RNAs. The results on both the phosphorus reconstructions and the total reconstructions (protein and ribosomal RNA) reveal several passageways through both subunits. The structures correspond favourably with other independent reconstructions of the whole E. coli ribosome from cryoelectron micrographs and their accompanying models of translation (Frank et al ., Nature , 376, 441–444, 1995; Stark et al ., Structure , 3, 815–821, 1995). The overall reconstructions in conjunction with the phosphorus (rRNA) distributions are the first to be achieved synchronously for this nucleoprotein complex.     

三七根际链霉菌Streptomyces sp.YNUCC0233木聚糖酶性质研究

从三七根际分离到一株产木聚糖酶链霉菌Streptomycessp.YNUCC0233。该菌所产生的木聚糖酶的最适反应温度为67℃,最适pH为6.0,在pH5.0~9.0和60℃以下时相对稳定。Ba2 和K 对该木聚糖酶有较强的促进作用,Hg2 、Cu2 、Mn2 、Co2 、Ni2 和EDTA对该酶有较强的抑制作用。对Streptomycessp.YNUCC023316SrDNA的1105bp片段的序列分析结果表明,Streptomycessp.YNUCC0233的16SrDNA片段与GenBank中所有已注册的链霉菌分类单位均不同。     

Fragments of rDNA Genes Scattered over the Human Genome Are Targets of Small RNAs

Small noncoding RNAs of different origins and classes play several roles in the regulation of gene expression. Here, we show that diverged and rearranged fragments of rDNA units are scattered throughout the human genome and that endogenous small noncoding RNAs are processed by the Microprocessor complex from specific regions of ribosomal RNAs shaping hairpins. These small RNAs correspond to particular sites inside the fragments of rDNA that mostly reside in intergenic regions or the introns of about 1500 genes. The targets of these small ribosomal RNAs (srRNAs) are characterized by a set of epigenetic marks, binding sites of Pol II, RAD21, CBP, and P300, DNase I hypersensitive sites, and by enrichment or depletion of active histone marks. In HEK293T cells, genes that are targeted by srRNAs (srRNA target genes) are involved in differentiation and development. srRNA target genes are enriched with more actively transcribed genes. Our data suggest that remnants of rDNA sequences and srRNAs may be involved in the upregulation or downregulation of a specific set of genes in human cells. These results have implications for diverse fields, including epigenetics and gene therapy.