Postcolonial Identities in Africa

Postcolonial Identities in Africa. Richard Werbner and Terence Ranger. eds. London: Zed Books, 1996. 292 pp.     

Initiation and maintenance of long term somatic embryogenesis from anthers and ovaries of Vitis longii ‘Microsperma’

Anthers and ovaries of Vitis longii Microsperma produced embryogenic callus when cultured on solidified Murashige and Skoog medium with 5M 2,4-dichlorophenoxyacetic acid (2,4-D) and 1M benzyladenine (BA). The initial callus was short-lived. However, long-term embryogenesis from callus was maintained through serial transfers by careful selection of clustered embryos with subtending callus. Alternatively, long term culture maintenance was through secondary embryogenesis which occurred directly from previously formed embryos on medium lacking growth regulators. Somatic embryos were white, exhibited frequent pluricotyly and tended to be larger than zygotic embryos. Histology of embryogenic callus demonstrated the presence of lipid-like substances and abundant starch. Somatic embryos were attached to callus by narrow to wide suspensor-like structures and possessed typical epidermal, cortical, and vascular tissue. Embryo cells contained abundant lipid-like accumulations but no starch. Embryos germinated when placed on medium containing 1M BA and produced plants of normal appearance.     

Release of a chimeric protein into the medium from Escherichia coli using the C-terminal secretion signal of haemolysin.

Recently, we have identified a novel topogenic sequence at the C terminus of Escherichia coli haemolysin (HlyA) which is essential for its efficient secretion into the medium. This discovery has introduced the possibility of using this secretion system for the release of chimeric proteins from E. coli directly into the medium. We have now successfully fused this C-terminal signal to a hybrid protein containing a few residues of beta-galactosidase and the majority of the E. coli outer membrane porin OmpF lacking its own N-terminal signal sequence. We find that this chimeric protein is specifically translocated across the inner and outer membranes and is released into the medium. In addition, we have further localized the HlyA secretion signal to the final 113 amino acids of the C terminus. In fact, a specific secretion signal appears to reside at least in part within the last 27 amino acids of HlyA.     

Structural analysis of the temperature-sensitive mutant of bacteriophage T4 lysozyme, glycine 156----aspartic acid

The structure of the mutant of bacteriophage T4 lysozyme in which Gly-156 is replaced by aspartic acid is described. The lysozyme was isolated by screening for temperature-sensitive mutants and has a melting temperature at pH 6.5 that is 6.1 degrees C lower than wild type. The mutant structure is destabilized, in part, because Gly-156 has conformational angles (phi, psi) that are not optimal for a residue with a beta-carbon. High resolution crystallographic refinement of the mutant structure (R = 17.7% at 1.7 A resolution) shows that the Gly----Asp substitution does not significantly alter the configurational angles (phi, psi) but forces the backbone to move, as a whole, approximately 0.6 A away from its position in wild-type lysozyme. This induced strain weakens a hydrogen bond network that exists in the wild-type structure and also contributes to the reduced stability of the mutant lysozyme. The introduction of an acidic side chain reduces the overall charge on the molecule and thereby tends to increase the stability of the mutant structure relative to wild type. However, at neutral pH this generalized electrostatic stabilization is offset by specific electrostatic repulsion between Asp-156 and Asp-92. The activity of the mutant lysozyme is approximately 50% that of wild-type lysozyme. This reduction in activity might be due to introduction of a negative charge and/or perturbation of the surface of the molecule in the region that is assumed to interact with peptidoglycan substrates.     

The effect of 2,3-diphosphoglycerate on the tetramer-dimer equilibrium of carbon monoxide hemoglobin in dilute solution. Correlation between sedimentation and kinetic behavior

The effect of 2,3-diphospho-D-glycerate on the sedimentation coefficient of carbon monoxide hemoglobin was correlated with the fraction of rapidly reacting hemoglobin observed subsequent to flash photolysis at 23 degrees C at pH 7.30 in buffers of 0.1 M ionic strength. Concentrations of the organic phosphate up to about 5 mM resulted in an increase in S20,w, consistent with an increase in the fraction of tetrameric hemoglobin. A decrease in rapidly reacting hemoglobin parallelled the increase in the sedimentation coefficient. Between 5 and 20 mM 2,3-diphosphoglycerate, S20,w decreased, suggesting that dissociation to dimers was enhanced. An increase in rapidly reacting hemoglobin was also observed in this concentration range. Similar sedimentation results were obtained with oxyhemoglobin at pH 7.00 and carbon monoxide hemoglobin at pH 7.06. Assuming single binding sites on each species, the dissociation constants for 2,3-diphosphoglycerate binding to tetrameric and dimeric HbCO are 0.2-0.3 mM and 2-5 mM at pH 7.30. This biphasic effect of this physiologically important organic phosphate on the state of aggregation of R state hemoglobin has not been previously reported, but it is similar to that previously noted with inositol hexaphosphate, which enhanced tetramer formation at low concentrations, while at higher concentrations it promoted hemoglobin dissociation to dimers (White, S. L. (1976) J. Biol. Chem. 251, 4763-4769; Gray, R. D. (1980) J. Biol. Chem. 255, 1812-1818).