A model for cell type-specific differential gene expression during heterocyst development and the constitution of aerobic nitrogen fixation ability inAnabaena sp. strain PCC 7120

When deprived of combined nitrogen, aerobically-grown filaments ofAnabaena sp. strain PCC7120 differentiate specialized cells called the heterocysts. The differentiation process is an elaborate and well orchestrated programme involving sensing of environmental and developmental signals, commitment of cells to development, gene rearrangements, intricate DNA-protein interactions, and differential expression of several genes. It culminates in a physiological division of labour between heterocysts, which become the sole sites of aerobic nitrogen fixation, and vegetative cells, that provide photosynthate to the heterocysts in return for nitrogen supplies. We propose a model, to describe the chronology of the important events and to explain how cell type-specific differential gene expression is facilitated by DNA-protein interactions leading to the development of heterocysts and constitution of nitrogen-fixing apparatus inAnabaena.     

A 43 kDa DNA binding protein from the pea chloroplast interacts with and stimulates the cognate DNA polymerase.

A DNA binding protein with DNA polymerase 'accessory activity' has been identified and purified to apparent homogeneity from pea chloroplasts. This protein consists of a single subunit of 43 kDa and binds to DNA regardless of its base sequence and topology. It increases cognate DNA polymerase-primase activity in a dose dependent manner. Using solid phase protein-protein interaction trapping and co-immunoprecipitation techniques, the purified protein was found to associate with the chloroplast DNA polymerase. The chloroplast DNA polymerase also binds directly to the radioiodinated 43 kDa protein. The specific interaction between 43 kDa protein and chloroplast DNA polymerase results in the synthesis of longer DNA chains. The 43 kDa protein, present abundantly in the pea chloroplast, appears to increase processivity of the chloroplast DNA polymerase and may play an important role in the replication of pea chloroplast DNA.     

Approximate methods for estimating the pattern of nucleotide substitution and the variation of substitution rates among sites

We propose two approximate methods (one based on parsimony and one on pairwise sequence comparison) for estimating the pattern of nucleotide substitution and a parsimony-based method for estimating the gamma parameter for variable substitution rates among sites. The matrix of substitution rates that represents the substitution pattern can be recovered through its relationship with the observable matrix of site pattern frequences in pairwise sequence comparisons. In the parsimony approach, the ancestral sequences reconstructed by the parsimony algorithm were used, and the two sequences compared are those at the ends of a branch in the phylogenetic tree. The method for estimating the gamma parameter was based on a reinterpretation of the numbers of changes at sites inferred by parsimony. Three data sets were analyzed to examine the utility of the approximate methods compared with the more reliable likelihood methods. The new methods for estimating the substitution pattern were found to produce estimates quite similar to those obtained from the likelihood analyses. The new method for estimating the gamma parameter was effective in reducing the bias in conventional parsimony estimates, although it also overestimated the parameter. The approximate methods are computationally very fast and appear useful for analyzing large data sets, for which use of the likelihood method requires excessive computation.      

Plant regeneration via somatic embryogenesis in chickpea (Cicer arietinum L.)

Efficient plant regeneration via somatic embryogenesis has been developed in chickpea cultivar C235. Leaf explants, on MS medium supplemented with 1.25 mg/l 2,4-D and 0.25 mg/l kinetin, yielded somatic embryos with high efficiency during dark incubation. MS medium supplemented with B₅ vitamins, 0.125 mg/l IBA and 2 mg/l BAP was found suitable for embryo maturation. The well formed embryos germinated into plantlets on basal B₅ medium supplemented with 0.25 mg/l BAP. Further development into healthy plantlets was obtained on basal B₅ medium. Hardened plantlets produced normal, fertile plants upon transfer to soil.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - BAP 6-Benzyl-aminopurine - IAA IndoIe-3-acetic acid - IBA Indole-3-butyric acid - NAA 1-Naphthalene acetic acid - Kinetin 6-furfuryl aminopurine - Zeatin 6-(4-hydroxy-3-methylbut-2-enylamino)-purine     

Inhibition of G1 phase cyclin dependent kinases by transforming growth factor β1

Transforming growth factor β1 (TGFβ1) inhibits epithelial cell proliferation late in the G1 phase of the cell cycle. We examined the effect of TGFβ1 on known late G1 cell cycle regulators in an attempt to determine the molecular mechanism of growth inhibition by this physiological inhibitor. The results demonstrate the TGFβ1 inhibits the late G1 and S phase specific histone H1 kinase activity of p33^cdk2. This inhibitiion is not dur to TGFβ1's effect on p33^cdk2 synthesis, but rather due to its negative effect on the late G1 phosphorylation of p33^cdk2. It is also shown that TGFβ1 inhibits both late G1 cyclin A and cyclin E associated histon H1 kinase activities. The inhibitor has no effects on the synthesis of cyclin E but to inhibit the synthesis of cyclin A protein in a cell cycle dependent manner. If TGFβ1 is added to cells which have progressed futher than 8 hours into G1, then it is without inhibitory effect on cyclin A synthesis. These effect on TGFβ1 on late G1 cell cycle regulators correlate well with its inhibitory effects on cellular growth and suggest that these G1 cyclin dependent kinases might serve as targets for TGFβ1-mediated growth arrest.     

The dialectic between globalization and localization: Economic restructuring,women and strategies of cultural reproduction

Dialectical Anthropology -     

A broadly active viral inhibitor in human and animal organ extracts and body fluids

To help assess the possibility that a newly described viral inhibitor from cell cultures might play a natural defensive role in vivo, its distribution and concentration in human and animal organ extracts and body fluids were investigated. The concentration of the inhibitor was high in human liver, heart muscle, splenic extracts, and human serum and milk. The inhibitor in the body was indistinguishable from a previously described inhibitor produced in cell cultures that was characterized by broad antiviral activity, lack of target cell species specificity, lack of induction of stable antiviral activity in cells, rapid reversibility of antiviral action, prevention of virus attachment, and stability at 100 degrees C. Sixteen virus plaque reduction units of the inhibitor diminished the yield of poliovirus in vitro by more than 1000-fold. Additional evidence that contact-blocking viral inhibitor (CVI) inhibits vaccinia virus attachment to cells is presented. A role for the inhibitor in natural defense against viral infections is possible.     

Transbilayer distributions of red cell membrane phospholipids in unilamellar vesicles

The phospholipid organization in unilamellar vesicles comprised of various purified phospholipid components of monkey erythrocyte membrane was ascertained using phospholipase A2 and trinitrobenzenesulfonic acid as external membrane probes. The vesicles were formed by sonication or detergent dialysis and fractionated by centrifugation or gel permeation chromatography. Experiments were done to confirm that the phospholipase A2 treatments did not cause lysis or induce fusion of the vesicles. This enzyme hydrolysed only the glycerophospholipids in the outer surface of the vesicles. The amounts of the external phospholipids determined by this enzymatic method were verified using the chemical probe, trinitrobenzenesulfonic acid. The choline-containing phospholipids and phosphatidylethanolamine localized randomly in the two surfaces of sonicated vesicles (outer diameter, about 30 nm), whereas phosphatidylserine preferentially distributed in the inner monolayer. This phosphatidylserine asymmetry virtually disappeared in detergent dialysed vesicles (outer diameter, about 45 nm). Furthermore, inclusion of cholesterol in both the types of vesicles resulted in more random glycerophospholipid distributions across the plane of vesicles bilayer, presumably due to the cholesterol-induced increases in the size of vesicles. These results demonstrate that the transbilayer distribution of erythrocyte membrane phospholipids in unilamellar vesicles are controlled mainly by the surface curvature rather than by interlipid interactions, and therefore suggest that phospholipid-phospholipid and phospholipid-cholesterol interactions should not play any significant role in determining the membrane phospholipid asymmetry in red cells. It is proposed that this asymmetry primarily originates from differential bindings of phospholipids with membrane proteins in the two leaflets of the membrane bilayer.     

Metalloproteinase inhibitors from bovine cartilage and body fluids

Inhibitors of the mammalian metalloproteinases, collagenase, proteoglycanase and gelatinase were isolated from bovine cartilage (extracts and culture medium) and bovine amniotic fluid and serum. These inhibitors either bind or do not bind to concanavalin-A--Sepharose, with Mr (gel filtration) of about 30 000 and 20 000, respectively. Cartilage and chondrocyte culture media contained only concanavalin-A-binding inhibitors whereas cartilage extracts contained only a non-binding inhibitor: serum and amniotic fluid contained both forms of inhibitory activities. In moist biochemical respects, particularly in their abilities to inhibit metalloproteinases, all of the inhibitors were found to be similar. It is concluded that the forms of the inhibitors that differ in Mr may be closely related to the tissue inhibitor of metalloproteinases (TIMP) previously purified from rabbit and human sources. These findings help to clarify other studies on collagenase inhibitors and support the concept that TIMP-like inhibitors may be important in the control of connective tissue degradation.