The exosome of Trypanosoma brucei.

The yeast exosome is a complex of at least 10 essential 3'-5' riboexonucleases which is involved in 3'-processing of many RNA species. An exosome-like complex has been found or predicted to exist in other eukaryotes but not in Escherichia coli. The unicellular parasite Trypanosoma brucei diverged very early in eukaryotic evolution. We show here that T.brucei contains at least eight exosome subunit homologs, but only a subset of these associate in a complex. Accordingly, the T.brucei exosome is smaller than that of yeast. Both free and complex-associated homologs are essential for cell viability and are involved in 5.8S rRNA maturation. We suggest that the exosome was present in primitive eukaryotes, and became increasingly complex during subsequent evolution.     

The influence of parasites on host sexual selection

In 1982 Hamilton and Zuk(1) proposed a provocative solution for the unexplained fact that the males of many species exhibit 'showy' traits such as brightly coloured plumage or vigorous courtship displays. They suggested that showy traits are fully expressed only by males who are resistant to parasites and that females examine such traits in order to choose resistant males as mates. Hamilton and Zuk's proposal has been the topic of extensive research and vigorous debate for nearly a decade. This article reviews the research, relevant criticisms and unanswered questions pertaining to the influence of parasites on sexual selection.     

Factors affecting zooplankton feeding by the sea anemoneAiptasia pallida

Effects of various treatments on prey capture, prey ingestion and ingestion time of individualArtemia salina nauplii by the sea anemoneAiptasia pallida Verrill were studied in the laboratory. Exposure to crudeArtemia homogenate, 5 × 10^–4 M reduced glutathione or 5 × 10^–4 M proline significantly decreased the number ofArtemia that were captured and ingested but had no significant effect on the ingestion time of individualArtemia. Multiple captures increased the total ingestion time but decreased ingestion time per prey item. Results suggest that, under these conditions, the prey capture phase of zooplankton feeding was somewhat distinct from the ingestion phase since chemical stimuli that significantly reduced prey capture had no significant effect on ingestion time.     

Chicken lens delta-crystallin gene expression and methylation in several non-lens tissues.

RNA sequences coding for the most abundant chicken lens proteins, delta-crystallin, were detected at very low levels in day old post hatch chick lung, heart, kidney and liver, and in 6 day embryo headless bodies. The pattern of cytosine methylation within the CCGG sequences of the delta-crystallin genes was also examined and shown to vary in several non-lens tissues, from several stages of development. Embryonic neural retina, which expresses a higher level of delta-crystallin RNA than the above tissues, is no less methylated in the sites studied than the tissues which have no association with the eye, and is actually more heavily methylated than the kidney. Thus no obvious correlation was found between undermethylation and gene expression.     

Nuclear RNase MRP is required for correct processing of pre-5.8S rRNA in Saccharomyces cerevisiae.

RNase MRP is a site-specific ribonucleoprotein endoribonuclease that cleaves RNA from the mitochondrial origin of replication in a manner consistent with a role in priming leading-strand DNA synthesis. Despite the fact that the only known RNA substrate for this enzyme is complementary to mitochondrial DNA, the majority of the RNase MRP activity in a cell is found in the nucleus. The recent characterization of this activity in Saccharomyces cerevisiae and subsequent cloning of the gene coding for the RNA subunit of the yeast enzyme have enabled a genetic approach to the identification of a nuclear role for this ribonuclease. Since the gene for the RNA component of RNase MRP, NME1, is essential in yeast cells and RNase MRP in mammalian cells appears to be localized to nucleoli within the nucleus, we utilized both regulated expression and temperature-conditional mutations of NME1 to assay for a possible effect on rRNA processing. Depletion of the RNA component of the enzyme was accomplished by using the glucose-repressed GAL1 promoter. Shortly after the shift to glucose, the RNA component of the enzyme was found to be depleted severely, and rRNA processing was found to be normal at all sites except the B1 processing site. The B1 site, at the 5' end of the mature 5.8S rRNA, is actually composed of two cleavage sites 7 nucleotides apart. This cleavage normally generates two species of 5.8S rRNA at a ratio of 10:1 (small to large) in most eukaryotes. After RNase MRP depletion, yeast cells were found to have almost exclusively the larger species of 5.8S rRNA. In addition, an aberrant 309-nucleotide precursor that stretched from the A2 to E processing sites of rRNA accumulated in these cells. Temperature-conditional mutations in the RNase MRP RNA gene gave an identical phenotype.Translation in yeast cells depleted of the smaller 5.8S rRNA was found to remain robust, suggesting a possible function for two 5.8S rRNAs in the regulated translation of select messages. These results are consistent with RNase MRP playing a role in a late step of rRNA processing. The data also indicate a requirement for having the smaller form of 5.8S rRNA, and they argue for processing at the B1 position being composed of two separate cleavage events catalyzed by two different activities.     

Morphotypes of friable embryogenic maize callus

Friable, embryogenic callus cultures of maize are normally maintained as a heterogeneous mixture of various morphotypes in different stages of development (Fransz and Schel 1991). With stringent selection during subculture, three Type II callus morphotypes have been enriched and maintained. Cultured tissue segments from the three morphotypes referred to as pre-embryogenic, early embryogenic, and late embryogenic were analyzed using light and scanning electron microscopy. The results suggest a developmental relationship between the three morphotypes. Regeneration studies substantiated that all morphotypes were capable of regenerating through somatic embryogenesis.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - BAP 6-benzylaminopurine - MS Murashige and Skoog(1962) Medium - SEM Scanning Electron Microscopy - FAA Formalin-acetic acid-alcohol     

The annual lipid cycle and feeding behavior of Alaskan redpolls

Summary Total lipids were extracted from 161 redpolls (Acanthis spp.) collected each month of the year from October 1962 to September 1963, in interior Alaska. A lipid index (Weight of ether extract x100/live body weight) was calculated for each sample. Lipids were also extracted from sections of pectoral muscle, livers and hearts representing each month.Body weight and lipid index were significantly positively correlated being highest in January and lowest in September. Total lipid content was significantly inversely correlated with air temperature; the high autumn and spring pre-migratory lipid peaks of migratory species were only weakly expressed in the redpolls. Liver lipid showed a significant annual variation being highest in December and lowest in August, while lipid from heart and pectoral muscle did not vary seasonally.Five birds were held in captivity during spring and summer at a constant temperature of 22°C. Food consumption was 5.1 g/day or 22.4 kcal. The caloric value of the most extensively utilized natural food, birch seed (Betula papyrifera), was determined (5.4–5.5 kcal/g dry wt). When esophageal diverticulae are full (2.0 g wet wt) of birch seeds, the resulting energy yield may sustain an individual for only a fraction of a 24 h winter day in contrast to other arctic herbivores (e.g. ptarmigan, Lagopus sp.) in which a full crop may suffice for the full 24 h period.     

Ageing in the chick lens: in vitro studies.

In principle, ageing may be due to the interaction of several factors, including the accumulation of random changes both genomic and non-genomic, secondary changes in a tissue contingent upon the changing function of other tissues, and programmed non-random changes in the tissue-specific expression of various genes. The use of a single tissue comprising one cell type only, in which the major gene products are well defined, in which there is a well attested series of developmental and age-related changes in cell properties and gene expression and which can be studied and compared in vivo and in vitro, offers advantages for investigation of these questions. The vertebrate eye lens possesses these advantages. The crystallins (proteins expressed at super-abundant levels in the lens) are well characterised. The lens epithelial cells (LEC) grow readily and can differentiate into the lens fibre cells in vitro, and, finally, such terminally differentiated cells may also be derived, by a process of transdifferentiation, from neural retina cells (NRC) in vitro. Thus the effect on ageing changes of the tissue of origin may also be studied. This article reviews our previous studies on long-term changes in growth potential, differentiation capacity and crystallin expression of chick lens cells in ageing cultures, their overall similarity to events in vivo and the effect on ageing changes of genotypes affecting the growth rate. It presents new information on these genetic aspects, and on crystallin expression in long-term ageing cultures of transdifferentiated neural retina, and compares the behaviour of ageing chick lens cells with that reported for mammals.