Characterization ofR. fredii QB1130, a strain effective on commercial soybean cultivars

Summary Two rhizobial strains (QB1130 and C3A) from northeast China were identified asRhizobium fredii on the basis of growth rate, media acidification and growth on a wide range of carbon substrates. The strains were shown to be distinct from USDA 191 on the basis of plasmid number and size. Bothnif and commonnod genes were located on the 295 kb plasmid of strains QB1130 and USDA 191, while onlynif genes were identified on this plasmid in C3A. When used to inoculate four commercial soybean (Glycine max) cultivars, one of the strains (C3A) was found to be ineffective, while the other (QB1130) was at least as effective as USDA 191, a strain ofR. fredii reported to be widely effective on North American cultivars of soybean. Further, QB1130 was capable of more effective nodulation of cowpea or the uncultivated soybean line, Peking, than either USDA 191 or the slow-growingBradyrhizobium japonicum USDA 16. Strain QB1130 should be useful for studies directed at improving symbiotic performance in soybean, or for studies of the comparative physiology and genetics of FG and SG strains on a single host.     

Subcellular localization of glycosidases and glycosyltransferases involved in the processing of N-linked oligosaccharides

Using isopycnic sucrose gradients, we have ascertained the subcellular location of several enzymes involved in the processing of the N-linked oligosaccharides of glycoproteins in developing cotyledons of the common bean, Phaseolus vulgaris. All are localized in the endoplasmic reticulum (ER) or Golgi complex as determined by co-sedimentation with the ER marker, NADH-cytochrome c reductase, or the Golgi marker, glucan synthase I. Glucosidase activity, which removes glucose residues from Glc₃Man₉(GlcNAc)₂, was found exclusively in the ER. All other processing enzymes, which act subsequent to the glucose trimming steps, are associated with the Golgi. These include mannosidase I (removes 1-2 mannose residues from Man_6-9[GlcNAc]₂), mannosidase II (removes mannose residues from GlcNAcMan₅[GlcNAc]₂), and fucosyltransferase (transfers a fucose residue to the Asn-linked GlcNAc of appropriate glycans). We have previously reported the localization of two other glycan modifying enzymes (GlcNAc-transferase and xylosyltransferase activities) in the Golgi complex. Attempts at subfractionation of the Golgi fraction on shallow sucrose gradients yielded similar patterns of distribution for all the Golgi processing enzymes. Subfractionation on Percoll gradients resulted in two peaks of the Golgi marker enzyme inosine diphosphatase, whereas the glycan processing enzymes were all enriched in the peak of lower density. These results do not lend support to the hypothesis that N-linked oligosaccharide processing enzymes are associated with Golgi cisternae of different densities.     

Radiobiology of ultrasoft X rays. I. Cultured hamster cells (V79)

Ultrasoft X rays (approximately less than keV) provide a useful probe for the study of the physical parameters associated with the induction of biological lesions because the spatial scale of their energy depositions is of nanometer dimensions, comparable to that of critical structures within the cell. We report on cell-killing experiments using cultured hamster cells (V79) exposed to carbon K (0.28 keV), aluminum K (1.5 keV), copper K (8.0 keV), and 250 kVp X rays, under oxic and hypoxic conditions, and as a function of cell-cycle phase. Our principal results are: RBE increases with decreasing X-ray energy; OER decreases with decreasing X-ray energy; and cell-cycle response is similar for all X-ray energies. Our RBE results confirm earlier observations using ultrasoft X rays on mammalian cells. The shapes of fitted curves through the data for each energy are statistically indistinguishable from one another, implying that the enhanced effectiveness is purely dose modifying. The results reported herein generally support the view that single-track effects of radiation are predominantly due to very local energy depositions on the nanometer scale, which are principally responsible for observed radiobiological effects.     

Further evidence from sectioned material in support of the existence of a linear terminal complex in cellulose synthesis

Summary Transmembrane linear terminal complexes considered to be involved in the synthesis of cellulose microfibrils have been described in the plasma membrane ofBoergesenia forbesii. Evidence for the existence of these structures has been obtained almost exlusively using the freeze etching technique. In the present study an attempt has been made to complete these studies using conventional fixation, staining, and sectioning procedures. In developing cells ofBoergesenia forbesii, strongly stained structures traversing the plasma membrane and averaging 598.9 nm ± 171.3 nm in length, 28.7 nm ± 4.2 nm in width, and 35.2 nm ± 6.6 nm in depth have been demonstrated. These structures are considered to be linear terminal complexes. At their distal (cell wall) surface, they appear to be closely associated with cellulose microfibrils. At the proximal (cytoplasmic) surface, they are associated with microtubules and polysomes. A model of the possible interrelation of the terminal complexes and microtubules leading to the generation of cell wall microfibrils is proposed.     

Junctions between lens cells in differentiating cultures: structure, formation, intercellular permeability, and junctional protein expression

We previously described cultures of chick embryo lens cells which displayed a marked degree of differentiation. In this report, the junctions found between the lens fiber-like cells in the differentiated "lentoids" are characterized in several ways. Thin-section methods with electron microscopy first demonstrated that numerous, large junctions between lentoid cells accompanied the other differentiated features of these cells. Freeze-fracture techniques, including quantitative analysis, then revealed that (a) junctional particles were loosely arranged as is typical of fiber cells, (b) the population of individual junctional areas in culture was indistinguishable from that found in 10- to 12-day chick embryo lenses, and (c) apparent junction formation occurred during the development of the lens cells, with lacy arrays of particles being associated with fiber-like junctions. In addition, gap junctions with hexagonally packed particles, typical of lens epithelial cells, largely disappeared during the course of differentiation. Injection of tracer dyes into lentoid cells resulted in rapid intercellular movement of dye, consistent with functional cell-to-cell channels connecting lentoid cells. During the development of the lens cells in culture, as junction formation occurred, an increase of approximately eight-fold in MP28 protein was observed within the cells. These combined results indicate that (a) extensive lens fiber junctions and functional cell-to-cell channels are found between differentiated lentoid lentoid cells in vitro, (b) lens fiber junctions appear to form during the course of lens cell differentiation in culture, (c) a significant increase occurs in the putative junctional protein before the cultures are highly developed, (d) the increased levels of MP28 and junction formation may be required for the full expression of the differentiated state in the lens fiber cell, and (e) this culture system should prove to be valuable for additional experiments on lens junctions and for other studies requiring the development of lens fiber cells in vitro.     

Molecular cloning of Clostridium difficile toxin A gene fragment in lambda gt11

Toxin A of Clostridium difficile has been purified and monospecific antiserum produced. A reliable procedure for isolation and restriction of C. difficile chromosomal DNA was developed which allowed for the construction of a genomic library in lambda gt11. Approx. 35,000 plaques were screened using anti-toxin A which resulted in the identification of one stable positive clone, lambda cd19. Verification of the immunological identity of the isolated toxin A gene fragment in lambda cd19 was determined by affinity purifying toxin A antibodies specific for lambda cd19 gene product, and using these selected antibodies to probe a Western blot of purified toxin A. The insert in lambda cd19 was demonstrated to be a 0.3 kb fragment by restriction digestion, and by hybridization of the clone to a chromosomal digest of C. difficile. The peptide coded for by the toxin A gene fragment in lambda cd19 was not cytotoxic for 3T3 mammalian tissue culture cells.     

Primary structure of an analog of crustacean pigment-dispersing hormone from the lubber grasshopper Romalea microptera

An octadecapeptide capable of inducing pigment dispersion in the chromatophores of the fiddler crab Uca pugilator has been isolated from lyophilized heads of the lubber grasshopper Romalea microptera. This pigment-dispersing factor (PDF) was purified by gel filtration, ion-exchange chromatography, partition chromatography, and reversed-phase high performance liquid chromatography. Automated gas-phase sequencing, followed by the identification of the carboxyl-terminal amide, established the primary structure of this PDF as Asn-Ser-Glu-Ile-Ile-Asn-Ser-Leu-Leu-Gly-Leu-Pro-Lys-Leu-Leu-Asn-Asp-Ala- NH2. This structure was confirmed by chemical synthesis and by demonstrating that the synthetic and native PDF displayed identical chromatographic behavior and biological activity. The Romalea PDF is structurally related to the crustacean pigment-dispersing hormones (PDHs), which are also octadecapeptides. The sequence of grasshopper PDF shows 78% homology with beta-PDH (from the crabs U. pugilator and Cancer magister) and 50% homology with alpha-PDH (from the prawn Pandalus borealis). This study provides the first direct chemical evidence for the structural relatedness of insect PDF to the crustacean PDHs, thus identifying them as an authentic family of arthropod peptides.     

Partial repair of deamidation-damaged calmodulin by protein carboxyl methyltransferase

Modification of calmodulin by protein carboxyl methyltransferase requires deamidation of one or more labile asparagine residues (Johnson, B.A., Freitag, N. E., and Aswad, D. W. (1985) J. Biol. Chem. 260, 10913-10916). We now show that deamidation results in the generation of two altered forms of calmodulin, designated A and B, which can be separated by electrophoresis under nondenaturing conditions. The A form is characterized by a larger apparent molecular radius, has only 10% the activity of native calmodulin when assayed for its ability to activate a Ca2+/calmodulin-dependent protein kinase from rat brain, and serves as an excellent substrate for the methyltransferase. The B form more closely resembles native calmodulin: it has an apparent molecular radius more like the native, exhibits about 40% the activity of native calmodulin, and is a relatively poor methyl acceptor. Evidence suggests that the A and B forms probably contain isoaspartate (A) and aspartate (B) in place of Asn-60 and/or Asn-97. Incubation of the A form with methyltransferase and S-adenosyl-L-methionine converts about half of the A form to an electrophoretic band indistinguishable from the B form. The activity of this partly converted calmodulin rises to 30-50% that of native calmodulin. These observations imply that the methyltransferase may have a biological role in restoring activity to proteins which contain abnormal isoaspartyl peptide bonds resulting from asparagine deamidation.     

Allosteric behavior in transducin activation mediated by rhodopsin. Initial rate analysis of guanine nucleotide exchange

Photolyzed rhodopsin acts in a catalytic manner to mediate the exchange of GTP for GDP bound to transducin. We have analyzed the steady-state kinetics of this activation process in order to determine the molecular mechanism of interactions between rhodopsin, transducin, and guanine nucleotides. Initial velocities (Vo) of the exchange reaction catalyzed by rhodopsin were measured for various transducin concentrations at several fixed levels of the GTP analog, [35S]guanosine 5'-(3-O-thio)triphosphate (GTP gamma S). The initial rate data analysis rigorously demonstrates that rhodopsin mediates the activation of transducin by a double-displacement catalytic mechanism. The Michaelis-Menten curves determined as a function of [transducin] reveal remarkable allosteric behavior; analysis of this data yields a Hill coefficient of 2. Lineweaver-Burk plots of Vo-1 versus [transducin]-1 display curvilinearity indicative of positive cooperativity and a series of parallel lines are generated by plotting Vo-1 as a function of [transducin]-2. The plots of Vo-1 versus [GTP gamma S]-1 show no evidence of allosterism and are a parallel series. Furthermore, the allosteric behavior observed in the activation of transducin is also witnessed in the rhodopsin-catalyzed guanine nucleotide exchange of the G protein's purified alpha subunit in the absence of the beta X gamma subunit complex. The latter observation implies that the molecular basis for allosterism in the activation process resides in the interactions between the photoreceptor and transducin's alpha subunit.