The physical maps for sequencing human chromosomes 1, 6, 9, 10, 13, 20 and X

We constructed maps for eight chromosomes (1, 6, 9, 10, 13, 20, X and (previously) 22), representing one-third of the genome, by building landmark maps, isolating bacterial clones and assembling contigs. By this approach, we could establish the long-range organization of the maps early in the project, and all contig extension, gap closure and problem-solving was simplified by containment within local regions. The maps currently represent more than 94% of the euchromatic (gene-containing) regions of these chromosomes in 176 contigs, and contain 96% of the chromosome-specific markers in the human gene map. By measuring the remaining gaps, we can assess chromosome length and coverage in sequenced clones.     

Riparian vegetation along the middle Snake River, Idaho: zonation, geographical trends, and historical changes

A baseline study was conducted on an 83-km free-flowing reach of the Snake River between Swan Falls Dam and the Idaho-Oregon border. The research had 2 components: (1) field characterization and inventory of existing riparian flora, vegetation, and environment (soils, topography, streamflow), and (2) determination and mapping, using a geographic information system, of historic changes in riparian vegetation based on a time series (1938-39, 1957, 1969, 1987) of aerial photographs. The flora was diverse, with 185 species of vascular plants identified, 63 of which were exotics. Vegetation was structured vertically along the riverbank gradient into lifeform-defined habitat types: emergent, riparian shrub-forb, tree, transitional grass-shrub, and upland. Riverbank seepage, probably of agricultural origin, blurred zonation patterns on some sites and added species to the overall flora. Upstream-downstream differences existed in the physical characteristics and vegetation of river subreaches. Coverage of riparian woodland, island riparian and total riparian vegetation, and area of islands increased since the 1930s, with the greatest changes in the 1969-1987 interval. Possible contributing facts were (1) significant declines in annual minimum flows since the 1950s, (2) decreases in peak flows following the completion of Swan Falls Dam and some upstream dams since the 1920s, (3) introduction and spread of exotic tree species ( Elaeagnus angustifolia and Tamarix spp.) and (4) possible effects of intensive agriculture on river sediment load and soil nutrients. The introduction and proliferation of purple loosestrife ( Lythrum salicaria ) could have considerable future effects on vegetation-channel dynamics in the middle Snake River.     

Composition and abundance of periphyton and aquatic insects in Sierra Nevada, California, stream

The species composition of periphyton and benthic insect communities and abundances of common taxa (&;gt;0.1&;#37; of individuals) were examined during snow-free months in Convict Creek, a permanent snowmelt- and spring-fed stream in the Sierra Nevada of California. The communities were highly diverse. The most abundant taxa in the periphyton were diatoms ( Achnanthes minutissima, Cocconeis placentula lineata, Cymbella microcephala, C. sinuata, Fragilaria construens, F. crotonensis , Navicula spp., Synedra acus , and S. rumpens ), except in late spring and summer when several seasonal blue-green algae ( Chamaesiphon incrustans, Lyngbya spp. and Oscillatoria spp.) are at their maximum densities. Most common periphyton taxa vary systematically in abundance with season, but relative abundances of taxa also appear to be influenced by streambed scouring and by concentrations of ambient nutrients. Data on population densities and length frequencies of larval and nymphal stages of common benthic insects and occurrences of pupal and adult stages were examined to determine life history patterns. Taxa hatching in winter and spring and abundant as immatures in late spring include ephemeropterans ( Epeorus longimanus, Drunella flavilinea , and Caudatella heterocaudata ), plecopterans ( Calineuria californica, Doroneuria baumanni, and Pteronarcys princeps ) and dipterans ( Cryptolabis sp.). Common taxa hatching in late spring or summer are the plecopteran Malenka ( californica? ) and the trichopterans Arctopsyche grandis and Rhyacophila acropedes . Several bivoltine and multivoltine ephemeropterans ( Baetis devinctus and B. tricaudatus ) and dipterans ( Simulium spp. and Chironomidae) have summer cohorts. Taxa hatching in late summer or autumn and most abundant in autumn include ephemeropterans ( Baetis spp., Ephemerella infrequens, Epeorus dulciana, Ironodes lepidus , and Paraleptophlebia pallipes ), trichopterans ( Hydropsyche oslari, Lepidostoma spp., Glossosoma califica, Micrasema sp., Brachycentrus americanus, Neophylax sp., and Rhyacophila vaccua ) and dipterans ( Antocha monticola, Pericoma sp., and Chironomidae). Major recurring events that may influence life history patterns and structure of the benthic insect community are (1) near-freezing, nighttime winter water temperatures and occasional anchor ice, (2) a prolonged period of high discharge in late spring and early summer (3) a brief summer, and (4) a prolonged period of moderate stream discharge in autumn when the substratum is stable and food is abundant.     

A large increase in enzyme-substrate affinity by protein engineering

A single point mutation has been engineered in the tyrosyl-tRNA synthetase that improves its affinity (KM) for its substrate ATP by a factor of 100. In the crystal structure of the tyrosyl tRNA synthetase (of Bacillus stearothermophilus), the side-chain hydroxyl of Thr 51 appears to make a weak hydrogen bond with the AMP moiety of the substrate intermediate, tyrosyl adenylate. In the absence of substrate, however, the hydroxyl group should make a strong hydrogen bond with water which would favour dissociation of the enzyme-substrate complex. We have used oligodeoxynucleotide-directed mutagenesis to construct two point mutants at this site: one to remove the hydroxyl group (Thr 51 leads to Ala 51) and the other, in addition, to distort the local polypeptide backbone (Thr 51 leads to Pro 51). We report here that both mutants have increased activity (kcat/KM for ATP) but one mutant (Pro 51) shows a massive 25-fold increase due mainly to a lowered KM for ATP. This demonstrates dramatically the potential of in vitro mutagenesis for improving the affinity of an enzyme for its substrate.