Raf: the holy grail of Ras biology?

Ras proteins regulate cell growth and differentiation, and their mutation plays a major, direct role in causing human cancer. For years, their precise function has been a mystery. One of the pathways Ras controls has recently been identified. It consists of a cascade of kinases (Raf, MEK and MAP kinases) that transmits signals from the plasma membrane to the nucleus. The role of Ras is remarkably simple: it recruits Raf from the cytoplasm to the plasma membrane. Once in the membrane, Raf is activated and the kinase cascade is initiated. Is this the whole story?     

Big flies, small repeats: the "Thr-Gly" region of the period gene in Diptera

The region of the clock gene period (per) that encodes a repetitive tract of threonine-glycine (Thr-Gly) pairs has been compared between Dipteran species both within and outside the Drosophilidae. All the non- Drosophilidae sequences in this region are short and present a remarkably stable picture compared to the Drosophilidae, in which the region is much larger and extremely variable, both in size and composition. The accelerated evolution in the repetitive region of the Drosophilidae appears to be mainly due to an expansion of two ancestral repeats, one encoding a Thr-Gly dipeptide and the other a pentapeptide rich in serine, glycine, and asparagine or threonine. In some drosophilids the expansion involves a duplication of the pentapeptide sequence, but in Drosophila pseudoobscura both the dipeptide and the pentapeptide repeats are present in larger numbers. In the nondrosophilids, however, the pentapeptide sequence is represented by one copy and the dipeptide by two copies. These observations fulfill some of the predictions of recent theoretical models that have simulated the evolution of repetitive sequences.      

Algae as indicators of environmental change

Despite an increased awareness by governments and the general public of the need for protecting all types of aquatic habitats, human impacts continue to impair the services that these ecosystems provide. Increased monitoring activities that locus on all major biological compartments are needed to quantify the present condition of Earth's aquatic resources and to evaluate the effectiveness of regulations designed to rehabilitate damaged ecosystems. Algae are an ecologically important group in most aquatic ecosystems but are often ignored as indicators of aquatic ecosystem change. We attribute this situation both to an underappreciation of the utility of algal indicators among non-phycologists and to a lack of standardized methods for monitoring with algae.Because of their nutritional needs and their position at the base of aquatic foodwebs, algal indicators provide relatively unique information concerning ecosystem condition compared with commonly used animal indicators. Algae respond rapidly and predictably to a wide range of pollutants and, thus, provide potentially useful early warning signals of deteriorating conditions and the possible causes. Algal assemblages provide one of the few benchmarks for establishing historical water quality conditions and for characterizing the minimally impacted biological condition of many disturbed ecosystems. Preliminary comparisons suggest that algal indicators are a cost-effective monitoring tool as well.Based on available evidence from field studies, we recommend development of taxonomic indicators based on diatoms (Bacillariophyceae) as a standardized protocol for monitoring ecosystem change. Both population- and community-level indices have inherent strengths, and limitations and information from both levels of biological organization should be utilized in tandem. However, further information concerning species tolerances to a variety of anthropogenic stressors is needed if autecological indices are to be used routinely for monitoring purposes. While functional measures (e.g. productivity) may also prove useful as monitoring tools, further investigation is required to characterize the reliability of alternative methodologies and to assess the consistency of these indicators under varying field conditions.Author for correspondence     

A model for 8-10 Hz spindling in interconnected thalamic relay and reticularis neurons.

We investigated a simplified model of a thalamocortical cell and a reticular thalamic cell interconnected with excitatory and inhibitory synapses, based on Hodgkin-Huxley type kinetics. The intrinsic oscillatory properties of the model cells were similar to those observed from single cells in vitro. When synaptic interactions were included, spindle oscillations were observed consisting of sequences of rhythmic oscillations at 8-10 Hz separated by silent periods of 8-40 s. The model suggests that Ca2+ regulation of lh channels may be responsible for the waxing and waning of spindles and that the reticular cell shapes the 10-Hz rhythmicity. The model also predicts that the kinetics of gamma-aminobutyric acid inhibitory postsynaptic potentials as well as the intrinsic properties of reticular cells are critical in determining the frequency of spindle rhythmicity.     

Characterization of full-length neurofibromin: tubulin inhibits Ras GAP activity.

Full-length neurofibromin is a GTPase activating protein (GAP) for the Ras proto-oncogene product. Regulation of neurofibromin activity therefore has important implications for cell growth. Neurofibromin co-purifies with tubulin when expressed in insect cells. The interaction between neurofibromin and tubulin is sensitive to the microtubule depolymerizing agent colchicine. Neurofibromin GAP activity is inhibited even at low concentrations of tubulin. However, maximal inhibition of GAP activity is only approximately 70%, suggesting that the neurofibromin-tubulin complex retains residual GAP activity. This decreased activity is reflected by a 4-fold decrease in its affinity for Ras. A truncated mutant of neurofibromin with reduced sensitivity to tubulin localizes some tubulin-binding determinants to an 80 residue segment immediately N-terminal to the GAP-related domain. Since tubulin is an abundant protein in eukaryotic cells, the tubulin-neurofibromin interaction may regulate the Ras signalling pathway.     

Construction and characterization of a YAC library with a lowfrequency of chimeric clones from flow-sorted human chromosome 9

Human chromosome 9 DNA, flow-sorted from somaticcell hybrid PK-87-9, has been used to construct two complete digest YAC libraries. The combined representation of chromosome 9 in these libraries, estimated by hybridization of chromosome 9-specific sequences to YAC colony grids, is 95%. The frequency of chimeric clones, analyzed by fluorescence in situ hybridization of chromosome 9 YACs to human metaphase chromosomes, was estimated to be 4%. These libraries provide a resource for physical mapping and for moving from genetic markers to disease loci on chromosome 9.     

3D 13C/1H NMR-based assignments for side-chain resonances of Lactobacillus casei dihydrofolate reductase. Evidence for similarities between the solution and crystal structures of the enzyme

Summary ¹³C-based three-dimensional ¹H–¹H correlation experiments have been used to determine essentially complete ¹³C and ¹H resonance assignments for the amino acid side chains of uniformly ¹³C/¹⁵N labelled L. casei dihydrofolate reductase in a complex with the drug methotrexate. Excellent agreement is observed between these assignments and an earlier set of partial assignments made on the basis of correlating nuclear Overhauser effect and crystal structure data, indicating that the tertiary structure of the enzyme is similar in solution and in the crystal state.To whom correspondence should be addressed.     

A 610 kb YAC clone harbors 7 cM of tomato (Lycopersicon esculentum) DNA that includes themale sterile 14 gene and a hotspot for recombination

Pollen development requires both sporophytic and gametophytic gene expression. We are using a map-based cloning technique to isolate sporophytic genes which, when mutant, cause pollen abortion and a male sterile (ms) phenotype in tomato (Lycopersicon esculentum). We have genetically characterized onems locus (ms14) using RFLP analysis and identified flanking markers. High-resolution genomic physical mapping indicates that thems14 locus is located in a ～300 kb region. We have identified a YAC clone with an insert size of ～610 kb that contains thems14-linked markers, reflects the organization of the physical map and therefore most probably contains thems14 gene. In addition, we present evidence that the relationship between physical and genetic distance in this chromosomal region changes abruptly from ～105–140 kb/cM to less than 24 kb/cM, and suggest that the TG393-TG104 region is a hotspot for recombination.