Deletion of the nuclear exosome component RRP6 leads to continued accumulation of the histone mRNA HTB1 in S-phase of the cell cycle in Saccharomyces cerevisiae

The nuclear exosome, a macromolecular complex of 3′ to 5′ exonucleases, is required for the post-transcriptional processing of a variety of RNAs including rRNAs and snoRNAs. Additionally, this complex forms part of a nuclear surveillance network where it acts to degrade any aberrantly processed mRNAs in the nucleus. The exosome complex has been implicated in the biogenesis pathway of general messenger RNAs through its interaction with the 3′-end processing machinery. During the cell cycle, yeast histone mRNAs accumulate in the S-phase and are rapidly degraded as cells enter the G2-phase. To determine if the exosome contributes to the cyclic turnover of yeast histone mRNAs, we examined the pattern of accumulation of ‘HTB1’ mRNA during the cell cycle in a deletion strain of ‘RRP6’, a component of the nuclear exosome. Our results show that cells lacking Rrp6p continue to accumulate HTB1 mRNA as the cell cycle proceeds. This continued accumulation appears to result from a delay in exit from S-phase in rrp6 cells. The accumulation of HTB1 mRNA in rrp6 cells is influenced by the interaction of the nuclear exosome with the 3′-end processing machinery although there is no evidence for differential regulation of histone mRNA 3′-end processing during the yeast cell cycle

Aspartoacylase-LacZ Knockin Mice: An Engineered Model of Canavan Disease

Canavan Disease (CD) is a recessive leukodystrophy caused by loss of function mutations in the gene encoding aspartoacylase (ASPA), an oligodendrocyte-enriched enzyme that hydrolyses N-acetylaspartate (NAA) to acetate and aspartate. The neurological phenotypes of different rodent models of CD vary considerably. Here we report on a novel targeted aspa mouse mutant expressing the bacterial β-Galactosidase (lacZ) gene under the control of the aspa regulatory elements. X-Gal staining in known ASPA expression domains confirms the integrity of the modified locus in heterozygous aspa lacZ-knockin (aspalacZ/+) mice. In addition, abundant ASPA expression was detected in Schwann cells. Homozygous (aspalacZ/lacZ) mutants are ASPA-deficient, show CD-like histopathology and moderate neurological impairment with behavioural deficits that are more pronounced in aspalacZ/lacZ males than females. Non-invasive ultrahigh field proton magnetic resonance spectroscopy revealed increased levels of NAA, myo-inositol and taurine in the aspalacZ/lacZ brain. Spongy degeneration was prominent in hippocampus, thalamus, brain stem, and cerebellum, whereas white matter of optic nerve and corpus callosum was spared. Intracellular vacuolisation in astrocytes coincides with axonal swellings in cerebellum and brain stem of aspalacZ/lacZ mutants indicating that astroglia may act as an osmolyte buffer in the aspa-deficient CNS. In summary, the aspalacZ mouse is an accurate model of CD and an important tool to identify novel aspects of its complex pathology

Biogenesis of histone mRNAs in the yeast Saccharomyces cerevisiae

THESIS 8384The typical eukaryotic human diploid cell contains 3.2x10 9 base pairs of deoxyribonucleic acid (DNA) which, if presented in an extended form, would measure 1.2m in length. The large amount of DNA is tightly wrapped and compacted through its interaction with a set of small basic proteins called histones (Alberts et al, 2002). Histones are positively charged proteins that facilitate the folding of DNA. Four types of histones have been described: H2A, H2B, H3 and H4 (Baxevanis and Landsman, 1997). A 147 base pair segment of eukaryotic DNA associates with histones, forming an octameric protein complex known as the nucleosome (Baxevanis and Landsman, 1998; Grunstein and Mann, 1992). The quaternary structure of the nucleosome is stabilised through an interaction with an additional histone, H1. This structure is highly conserved. The level of compaction of DNA varies during the life cycle of the cell. During Interphase, chromatin is relatively decondensed and distributed throughout the nucleus. As cells enter mitosis, chromatin condenses into chromosome structures, which are relatively transcriptionally silent. As the cells undergo DNA replication, the newly formed DNA must be repackaged into chromatin. Therefore the production of histones is tightly controlled to ensure that maximum production occurs In the S-phase of the cell cycle (Xu et al, 1990)

mRNA HTB1 in S-phase

leads to continued accumulation of the histon