The Mechanochemistry of Muscular Contraction : I. The isometric twitch

The dependence of PC¹ and ATP¹ dephosphorylation on the number of isometric twitches in the iodoacetate-nitrogen-poisoned muscle has been examined. There is no net dephosphorylation of adenosinetriphosphate. PC dephosphorylation varies linearly with the number of twitches and produces equivalent amounts of C¹ and P_1i.¹ Iodoacetate concentrations which block the enzyme, creatine phosphokinase, render the muscle non-contractile. A value of 0.286 µmole/gm. for the amount of PC split per twitch is obtained which gives a value of -9.62 kcal./mole for the "physiological" heat of hydrolysis of PC in agreement with expectations based on thermochemical data. In a single maximal isometric twitch it is estimated that 2 to 3 PC molecules are dephosphorylated per myosin molecule, or 1 per actin molecule. The results support the view that under the conditions of these experiments PC dephosphorylation is the net energy yielding reaction. The in vivo stoichiometry of the mechano-chemistry of contraction revealed by these studies on the one hand, and the known stoichiometry of actin polymerization and its coupling to the creatine phosphokinase system on the other are strikingly similar and strongly suggest that the reversible polymerization of actin is involved in a major way in the contraction-relaxation-recovery cycle of muscle.     

The preservation of the ability of cultured quail wing bud mesoderm to elicit a position-related differentiative response

A previous study showed that grafting wedges of fresh anterior quail wing mesoderm into posterior slits of chick wing buds resulted in the formation of rods and nodules of cartilage in a high percentage of cases (B. Carlson, 1983, Dev. Biol. 101, 97-105). The purpose of the present study was to determine if a similar response could be elicited by grafting pieces of mesoderm that had been cultured in vitro. When pieces of 1-day cultured anterior mesoderm from stage 17-24 donors were grafted into standard posterior slits of chick wing buds, the percentages of supernumerary structures differed little from those which formed after the grafting of pieces of fresh mesoderm. In a time series, grafts of stage 22-23 anterior mesoderm which had been cultured for 1-4 days retained the ability to form cartilage after being grafted into posterior locations. A time series showed that the duration of this retention was longer in cultured mesoderm than it was in mesoderm that remains in the donor wing bud.     

Anaphase chromosome movement in the unequally dividing grasshopper neuroblast and its relation to anaphases of other cells

The positions of the two sets of chromosome kinetochores, the spindle poles, cell membrane adjacent to the poles, and cleavage furrow of grasshopper neuroblasts in culture at 38°C were determined at short-time intervals during anaphase. The percent of motion due to poleward movement and spindle elongation, which coincide in time, were calculated for each minute, the former falling from 61% in the first minute to 15% in the seventh minute, and increasing to 86% in the final minute, probably as a result of pressure and bending of the spindle. Of the total chromosome movement during anaphase 44.6% is due to poleward movement of the daughter kinetochores and 55.4% to spindle elongation. The maximum velocity of a set of kinetochores is 3.41 m/min and the mean velocity 1.86 m/min (one-half the rate of separation). Various studies of anaphase chromosome movement in different cells and different species suggest certain generalizations, some of which are based on very small samples and so must be considered quite tentative: (1) The combination of poleward movement and spindle elongation is much more frequent than either acting alone. (2) These components of movement may coincide in time, overlap, or spindle elongation may follow poleward movement, but spindle elongation never begins before poleward chromosome movement. (3) There is an optimum temperature for the rate of chromosome movement, above and below which the rate gradually decreases. (4) In homoiothermic animals this optimum occurs at normal body temperature. (5) In homoiothermic animals the velocity falls more rapidly with a decrease in temperature than in poikilothermic animals. (6) Animals with large chromosomes (amphibia, grasshoppers) have higher chromosome velocities than those with small chromosomes. (7) Non-meiotic cells and secondary spermatocytes have higher velocities than primary spermatocytes of the same species. (8) Chromosome velocity is lower in malignant than non-malignant cells. (9) Chromosome velocity tends to be positively correlated with the distance the chromosomes travel during anaphase.     

Plants interact with microbial polysaccharides

Plants are resistant to almost all of the microorganisms with which they come in contact. In response to invasion by a fungus, bacterium, or a virus, many plants produce low molecular weight compounds, phytoalexins, which inhibit the growth of microorganisms. Phytoalexins are produced whether or not the invading microorganism is a pathogen. The production of phytoalexins appears to be a widespread mechanism by which plants attempt to defend themselves against pests. Molecules of microbial origin which trigger phytoalexin accumulation in plants are called elicitors. Structural polysaccharides from the mycelial walls of several fungi elicit phytoalexin accumlation in plants. Approximately 10 ng of the polysaccharide elicits the accumulation in plants of more than sufficient amounts of phytoalexin to stop the growth of microorganisms in vitro. The best characterized elicitors have been demonstrated to be β-1,3-glucans with branches to the 6 position of some of the glucosyl residues. Oligosaccharides, produced by partial acid hydrolysis of the mycelial wall glucans, are exceptionally active elicitors. The smallest oligosaccharide which is still an effective elicitor is composed of about 8 sugar residues. Bacteria also elicit phytoalexin accumulation in plants, but the Rhizobium symbionts of legumes presumably have a mechanism which allows them to avoid either eliciting phytoalexin accumulation or the effects of the phytoalexins if they are accumulated. The lectins of legumes bind to the lipopolysaccharides of their symbiont, but not of their non-symbiont, Rhizobium. It is not known whether the lectin-lipopolysaccharide interaction is involved with the establishment of symbiosis. However, evidence will be presented that suggests that lectins are, in fact, enzymes capable of modifying the structurs of the lipopolysaccharides of their symbiont, but not of their non-symbiont, Rhizobium. It will also be shown that the lipopolysaccharides isolated from different Rhizobium species and from different strains of individual Rhizobium species have different sugar compositions. Thus, the different strains of a single Rhizobium species are as different from one another as the different species of Salmonella and other gram-negative bacteria. This conclusion is substantiated by experiments demonstrating that antibodies to the lipopolysaccharide from a single Rhizobium strain can differentiate that strain from other strains of the same species as well as from other Rhizobium species. The role in symbiosis of the strain-specific O-antigens is unknown.