Malik Williams

The construction of rhizobial strains which increase plant biomass under controlled conditions has been previously reported. However, there is no evidence that these newly constructed strains increase legume yield under agricultural conditions. This work tested the hypothesis that carefully manipulating expression of additional copies of nifA and dctABD in strains of Rhizobium meliloti would increase alfalfa yield in the field. The rationale for this hypothesis is based on the positive regulatory role that nifA plays in the expression of the nif regulon and the fact that a supply of dicarboxylic acids from the plant is required as a carbon and energy source for nitrogen fixation by the Rhizobium bacteroids in the nodule. These recombinant strains, as well as the wild-type strains from which they were derived, are ideal tools to examine the effects of modifying or increasing the expression of these genes on alfalfa biomass. The experimental design comprised seven recombinant strains, two wild-type strains, and an uninoculated control. Each treatment was replicated eight times and was conducted at four field sites in Wisconsin. Recombinant strain RMBPC-2, which has an additional copy of both nifA and dctABD, increased alfalfa biomass by 12.9% compared with the yield with the wild-type strain RMBPC and 17.9% over that in the uninoculated control plot at the site where soil nitrogen and organic matter content was lowest. These increases were statistically significant at the 5% confidence interval for each of the three harvests made during the growing season. Strain RMBPC-2 did increase alfalfa biomass at the Hancock site; however, no other significant increases or decreases in alfalfa biomass were observed with the seven other recombinant strains at that site. At three sites where this experiment was conducted, either native rhizobial populations or soil nitrogen concentrations were high. At these sites, none of the recombinant strains affected yield. We conclude that RMBPC -2 can increase alfalfa yields under field conditions of nitrogen limitation, low endogenous rhizobial competitors, and sufficient moisture.

Abstract The incorporation of labeled amino acids into the α and β chains of hemoglobin has been studied in rabbit reticulocytes. Relative rates of synthesis were assessed by measurement of the specific activities of globin and of constituent α and β chains prepared from unpurified, ribosomefree hemolysates. The α:β specific activity ratio of approximately 1.0 indicated that labeled α and β chains were produced in nearly equal numbers. However, in hemoglobin which was purified from these hemolysates two features were noted that indirectly suggested the presence of a pool of α chains and a smaller pool of β chains: (a) the α:β specific activity ratio was less than 1.0, and (b) a loss of α chain radioactivity and a smaller loss of β chain radioactivity were observed following purification of the hemoglobin. More direct evidence for these previously formed pools has been found in (a) the radioactivity elution pattern observed on gel filtration of unpurified hemolysate, which revealed a radioactive protein of high specific activity, and (b) further analysis of this minor peak by ion exchange chromatography, peptide mapping, and spectrophotometry. Incubation with added hemin (1 x 10-4 m) was associated with (a) an increase in the specific activities of globin and of α and β chains, (b) the disappearance of the pool of β chains, which probably existed as αβ dimers, (c) persistence of a pool of α chains during the periods of incubation used, and (d) an increase in the α:β specific activity ratio in purified hemoglobin. A model of hemoglobin biosynthesis is suggested which could account for the observed effects of added hemin in terms of (a) stimulation of the synthesis of α and β chains, (b) combination of hemin with αβ dimers (globin) to form hemoglobin, and (c) promotion of the assembly of newly synthesized α and β chains, thus largely bypassing the pool of α chains. In this manner, heme may be said to coordinate as well as stimulate the synthesis of hemoglobin.

W. A. Thomas and M. K. Williams, J. Chem. Soc., Chem. Commun., 1972, 994 DOI: 10.1039/C39720000994

Cationic diblock copolymer spheres, worms and vesicles are synthesised <italic>via</italic> polymerisation-induced self-assembly; worms form soft, thermoresponsive anti-bacterial gels at 20 °C.