Electron-microscopic mapping of the hinge region of myosin

Summary The trypsin-sensitive sites in the labile hinge region of the myosin molecule are located with heightened accuracy (±2 nm) by electron microscopy as lying at 70, 85, 95, and 103 nm from the C-terminus of the rod section of the molecule.I thank Dr.Michael Young for valuable advice and encouragement, Drs.Jerome Gross andRomaine R. Bruns for use of their electron microscope, and Mrs.Muriel H. Blanchard for excellent technical assistance. This work was supported by the American Heart Association, The John A. Hartford Foundation, Inc., and the National Institutes of Health.     

Botanical content of black-tailed jackrabbit diets on semidesert rangeland

Botanical content of black-tailed jackrabbit diets was determined by microhistological examination of fecal samples collected from six different vegetation types in southern New Mexico on three dates. Grasses comprised the largest component of the jackrabbit diets, with dropseed species ( Sporobolus spp.) and black grama ( Bouteloua eriopoda ) the most abundant grasses in the diets. Leatherweed croton ( Croton pottsii ) and silverleaf nightshade ( Solanum elaeagnifolium ) were important forbs on most vegetation types. Diet composition varied in response to season and vegetation type. Grasses were important during the summer growing season, while forbs were selected during their growing season (summer or winter-spring). Shrubs were less abundant in the diet than grasses and forbs.     

The breeding system of an endangered pupfish ( Cyprinodon elegans )

We examined the breeding behavior of the endangered Comanche Springs pupfish ( Cyprinodon elegans ) during 3 separate breeding seasons in a pool of springwater originating in Phantom Cave, near Balmorhea State Park, Texas, in relation to behaviors reported for the species in swift-flowing canals (Itzkowitz 1969). In the quiet pool, unlike in swift water, the breeding system was characterized by 3 different male mating tactics: territorial defense, satellite positioning, and sneak spawning. Although the breeding strategies adopted by the males were conditional, mating tactics generally reflected male size. Territorial residents were the largest, satellites were medium-sized, and sneakers were the smallest adult males observed in the population. Territorial males traversed the least amount of area, defending compact territories, while satellites and sneakers covered more area in attempts to spawn. Although preferred territories in swift water centered on algal mats, territorial residents in quiet water seemed to prefer territories around large rocks. Males defending rocks had higher reproductive successes than males occupying other substrate types. The flexibility of the males' tactics was exhibited during one year of study when large males appeared to be absent from the population. During that year, medium-sized males switched from the satellite to territorial tactic, while small males remained sneakers.     

Occurrence, persistence, and expansion of saltcedar ( Tamarix spp.) populations in the Great Plains of Montana

Saltcedar ( Tamarix spp.), a shrub native to Eurasia, is associated with major alterations to wetland and riparian systems in the southwestern United States. Since the 1960s saltcedar has been naturalized in northern states of the U.S. where its growth potential and impacts are not well known. Here, we describe the occurrence, age, size, and relative cover of saltcedar populations in several river basins in central eastern Montana, USA, to identify potential patterns of spread across the region and changes in individual populations as they age. Stands were aged according to the oldest saltcedar individuals and were sampled for dominant plant cover and soil properties. Multiple introductions appear to have occurred in Montana, with the oldest stands occurring on the Bighorn River in southern Montana. Saltcedar absolute and relative cover and stand area increased significantly with stand age, while native tree and shrub relative cover remained low across all stand ages. These results suggest that saltcedar stands establish where woody natives are not abundant and that they persist and expand over time. Although soil salinity remained constant, soil pH decreased with saltcedar stand age, indicating a possible effect of organic matter inputs. An analysis of annual wood increment of saltcedar and sandbar willow (a native with analogous growth form) stems along a latitudinal gradient showed that stem growth of both species did not differ significantly among regions. Stem growth decreased inversely with elevation for both species while growth responses to elevation did not differ between species. Our results show an increase in number of populations and continued viability of these populations. Mechanisms of saltcedar increases in this region are yet to be determined. Anthropogenic influences, such as saltcedar plantings, watershed alterations (e.g., river flow control), and habitat disturbances (e.g., cattle grazing or habitat clearing) may facilitate its spread in similar climates of the Great Plains.     

Cyclin synthesis drives the early embryonic cell cycle

We have produced extracts of frog eggs that can perform multiple cell cycles in vitro. Destruction of the endogenous messenger RNA arrests the extracts in interphase. The addition of exogenous cyclin mRNA is sufficient to produce multiple cell cycles. The newly synthesized cyclin protein accumulates during each interphase and is degraded at the end of each mitosis.     

Cloning and expression of a complementary DNA encoding a bovine adrenal angiotensin II type-1 receptor

Angiotensin II elicits different responses which affect cardiovascular, neuronal and electrolyte transport regulation. To understand the mechanisms responsible for its various actions, the receptor for angiotensin II has long been sought, but numerous attempts to purify the receptor have been unsuccessful owing to its instability and low concentration. We report here the expression cloning of a complementary DNA encoding a bovine angiotensin II receptor to overcome these difficulties. The receptor cDNA encodes a protein of 359 amino-acid residues with a transmembrane topology similar to that of other G protein-coupled receptors. COS-7 cells transfected with the cDNA expressed specific and high-affinity binding sites for angiotensin II, angiotensin II antagonist and a non-peptide specific antagonist for type-1 receptor. Dithiothreitol inhibited ligand binding. The concentration of intracellular Ca2+ and of inositol-1,4,5-trisphosphate increased in the transfected COS-7 cells in response to angiotensin II or angiotensin III, indicating that this receptor is the type-1 receptor for angiotensin II. Northern blot analysis revealed that the messenger RNA for this receptor is expressed in bovine adrenal medulla, cortex and kidney.     

The formation of cubic ice under conditions relevant to Earth's atmosphere

An important mechanism for ice cloud formation in the Earth's atmosphere is homogeneous nucleation of ice in aqueous droplets, and this process is generally assumed to produce hexagonal ice. However, there are some reports that the metastable crystalline phase of ice, cubic ice, may form in the Earth's atmosphere. Here we present laboratory experiments demonstrating that cubic ice forms when micrometre-sized droplets of pure water and aqueous solutions freeze homogeneously at cooling rates approaching those found in the atmosphere. We find that the formation of cubic ice is dominant when droplets freeze at temperatures below 190 K, which is in the temperature range relevant for polar stratospheric clouds and clouds in the tropical tropopause region. These results, together with heat transfer calculations, suggest that cubic ice will form in the Earth's atmosphere. If there were a significant fraction of cubic ice in some cold clouds this could increase their water vapour pressure, and modify their microphysics and ice particle size distributions. Under specific conditions this may lead to enhanced dehydration of the tropopause region.     

Sequence and analysis of chromosome 5 of the plant Arabidopsis thaliana

The genome of the model plant Arabidopsis thaliana has been sequenced by an international collaboration, The Arabidopsis Genome Initiative. Here we report the complete sequence of chromosome 5. This chromosome is 26 megabases long; it is the second largest Arabidopsis chromosome and represents 21% of the sequenced regions of the genome. The sequence of chromosomes 2 and 4 have been reported previously and that of chromosomes 1 and 3, together with an analysis of the complete genome sequence, are reported in this issue. Analysis of the sequence of chromosome 5 yields further insights into centromere structure and the sequence determinants of heterochromatin condensation. The 5,874 genes encoded on chromosome 5 reveal several new functions in plants, and the patterns of gene organization provide insights into the mechanisms and extent of genome evolution in plants.