l h e multiple forms of branching enzyme (BE) from developing maize (Zea mays) endosperm were purified by modification of previous procedures such that amylase activity could be eliminated completely from the BE preparation. lhree distinct assays for BE activity (phosphorylase a stimulation assay, BE linkage assay, and iodine stain assay) were used to characterize and differentiate the properties of the BE isoforms. lhis study presents the first evidence that the BE isoforms differ in their action on amylopectin. BEI had the highest activity in branching amylose, but its rate of branching amylopectin was less than 5% of that of branching amylose. Conversely, BEll isoforms had lower rates in branching amylose (about 9-12% of that of BEI) and had higher rates of branching amylopectin (about 6-fold) than BEI. l h e implication of these findings to the mechanism of amylopectin synthesis in vivo are discussed.Starch biosynthesis is believed to occur through the enzyme reactions catalyzed by ADP-Glc pyrophosphorylase (EC 2.7.7.27), starch synthase (EC 2.4.1.21), and BE (EC 2.4.1.18) (Okita, 1992). Plant BEs, which catalyze the synthesis of the (1-6)-a-~-glucosidic linkage of amylopectin, are likely to play a key role in the regulation of starch quality (Preiss, 1991). Multiple forms of BE have been found in many plants, e.g. spinach leaf (Hawker et al., 1974), maize (Zea mays) leaf (Dang and Boyer, 1988), maize endosperm (Boyer and Preiss, 1978a), developing seeds of pea (Smith, 1988), rice (Nakamura et al., 1992), and germinating castor bean endosperm (Goldner and Beevers, 1989).One of the problems in studying BE is the difficulty in quantitatively assaying branching activity. BE activity is frequently assayed by monitoring the branching of amylose as measured by the decrease in absorbance of the amyloseiodine complex resulting from the branching (Borovsky et al., 1975;Boyer and Preiss, 1978a). BE can also be assayed by measuring its degree of stimulation of unprimed synthesis of a-D-glucan catalyzed by phosphorylase a from a-~-Glc-l-P (Hawker et al., 1974;Boyer and Preiss, 1978a). A quantitative method using reduced amylose has been developed recently to determine the number of branching linkages introduced by BE (Takeda et al., 1993). Amylolytic enzymes such as amylase interfere with a11 of these methods by hydrolyzing the a-D-glucan. Therefore, purification of BE away from Singh and Preiss (1985). Although the enzymes had been highly purified by those workers, some amylase activity was still present in their preparations. Here, we describe a modified procedure that leads to the purification of the multiple forms of BE that is virtually free of contaminating amylase. The three different assays for branching activity mentioned above were used to study and distinguish the properties of the BE isoforms present in the developing maize endosperm. In a previous paper (Takeda et al., 1993), we reported the characterization of the maize BE isoforms in branching amylose. The results obtained with the amylose indicated tha...

We describe a formulation for studying the quench dynamics of integrable systems generalizing an approach by Yudson. We study the evolution of the Lieb-Liniger model, a gas of interacting bosons moving on the continuous infinite line and interacting via a short range potential. The formalism allows us to quench the system from any initial state. We find that for any value of repulsive coupling independently of the initial state the system asymptotes towards a strongly repulsive gas, while for any value of attractive coupling, the system forms a maximal bound state that dominates at longer times. In either case the system equilibrates but does not thermalize. We compare this to quenches in a Bose-Hubbard lattice and show that there, initial states determine long-time dynamics independent of the sign of the coupling.

The gene encoding for mature branching enzyme (BE) I (BEI) of maize (Zea mays 1.) endosperm has been expressed in Escherichia co/i using the T7 promoter. The expressed BEl was purified to near homogeneity so that amylolytic activity and bacterial BE could be completely eliminated from the BE preparation. l h e recombinant enzyme showed properties very similar to those of BEI purified from developing maize endosperm with respect to branching amylose and amylopectin. This result confirmed our earlier report that maize endosperm BEI had a higher rate of branching amylose and a much lower rate (less than 10% of that of branching amylose) of branching amylopectin. This study also showed a great advantage in purifying BEI from the bacterial expression system rather than from developing maize endosperm. Most important, this study has established the system with which to study the structure-function relationships of the maize BEI using site-directed mutagenesis.

No abstract

Light/dark effects on growth and sugar accumulation in tomato (Lycoperskon esculentum) fruit during early development were studied on intact plants (In vivo) and in tissue culture (in vitro).Through the use of an in vitro culture of tomato fruit, it was possible to investigate the direct effects of light on sink metabolism by eliminating the source tissue. Similar growth patterns were found in vivo and in vitro. Fruit growth in different sugars indicated that sucrose was the best source of carbon for in vitro fruit growth. Fruit growth increased as sucrose concentration increased up to 8%. Darkening the fruit decreased fruit dry weight about 40% in vivo and in vitro. The differences in the CO2 exchange rate between light and dark grown fruit indicated that light stimulation of fruit growth was due to mechanisms other than photosynthesis. Supporting this conclusion was the fact that light intensities ranging from 40 to 160 micromoles per square meter per second had no significant influence on fruit growth, and light did not increase growth of fruit cultured with glucose or fructose as a carbon source. However, light stimulated fruit growth significantly when sucrose was used as the carbon source. Light-grown fruit took up 30% more sucrose from the same source and accumulated almost twice as much hexose and starch as dark-grown fruit. A possible expansion of an additional sink for carbon by light stimulation of starch synthesis during early development will be discussed.Assimilate partitioning and translocation are major determinants of crop yield. It has been suggested (7) that the regulation of these processes is at least partially through the metabolic activity of the sink tissue. As an example of this, it has been shown that in tomato (Lycopersicon esculentum), sucrose uptake can be manipulated by altering sink metabolism through an increase or decrease in temperature (5, 31). Furthermore, this type of research illustrates the potential usefulness of identifying and studying environmental factors influencing sink activity. From a practical standpoint, such information provides a basis for plant growth and environmental control models (1, 8, 18) increasingly used by greenhouse producers for regulating crop timing and yield. However, it also provides a point of attack in our quest to understand what "sink activity" actually means in biochemical terms.