Twintrons are not unique to the Euglena chloroplast genome: structure and evolution of a plastome cpn60 gene from a cryptomonad

Introns within introns (twintrons) are known only from the Euglena chloroplast genome. Twintrons are group II or III introns, into which another group II or III intron has been transposed. In this paper we describe a non-Euglena twintron structure within a plastid-encoded chaperone gene (cpn60) of the cryptomonad alga Pyrenomonas salina. In addition, the evolutionary relationships between members of the Cpn60 protein family are determined. Our findings permit the inclusion of cryptomonad plastomes in phylogenetic studies of intron evolution and present further evidence for the origin of modern plastids from a cyanobacterial ancestor.This paper is dedicated to Prof. Dr. Peter Sitte on the occasion of his 65th birthday     

A monoclonal antibody extends the half-life of an anti-HIV oligodeoxynucleotide and targets it to CD4+ cells.

An approach was sought to increase the half-life and target cell specificity of antisense oligodeoxynucleotides (oligos). A monoclonal antibody (MAb) was derived from mice immunised with an oligo complementary to a region (1-20) of the HIV genome. This MAb exerts a protective effect on the oligo from the degradation induced by plasma exonucleases in vitro and in vivo. Moreover the anti-oligo MAb dissociates from the oligo in the presence of its complementary sequence to allow hybridization of the two complementary strands. To direct the oligo to CD4+ cells the anti-oligo MAb was cross-linked to an anti-CD4 MAb. The heteroaggregate determines a 5-fold increase in the cellular membrane binding of the oligo to CD4+ lymphocytes. These findings suggest a new approach to enhancing the therapeutic action and the target specificity of antisense oligodeoxynucleotides useful for the selective inhibition of HIV replication in vivo.     

Innate colour preferences of flower visitors

Freshly emerged flower visitors exhibit colour preferences prior to individual experience with flowers. The understanding of innate colour preferences in flower visitors requires a detailed analysis, as, on the one hand, colour is a multiple-signal stimulus, and, on the other hand, flower visits include a sequence of behavioural reactions each of which can be driven by a preferential behaviour. Behavioural reactions, such as the distant approach, the close-range orientation, the landing, and the extension of mouthparts can be triggered by colour stimuli. The physiological limitations of spectral sensitivity, the neuro-sensory filters, and the animals' different abilities to make use of visual information such as brightness perception, wavelength-specific behaviour and colour vision shape colour preferences. Besides these receiverbased factors, there are restrictions of flower colouration due to sender-based factors such as the absorption properties of floral pigments and the dual function of flower colours triggering both innate and learned behaviour. Recordings of the spectral reflection of coloured objects, which trigger innate colour preferences, provide an objective measure of the colour stimuli. Weighting the spectral reflection of coloured objects by the spectral composition of the ambient light and the spectral sensitivity of the flower visitors' photoreceptors allows the calculation of the effective stimuli. Perceptual dimensions are known for only a few taxa of flower visitors.     

Nitrogenase — hydrogenase relationships in Rhizobium japonicum

The conditions necessary for coordinate derepression of nitrogenase and O₂-dependent hydrogenase activities in free-living cultures of Rhizobium japonicum were studied. Carbon sources were screened for their ability to support nitrogenase, and then hydrogenase activities. There was a positive correlation between the level of nitrogenase and corresponding hydrogenase activities among the various carbon substrates. The carbon substrate -ketoglutarate was able to support the highest levels of both nitrogenase and hydrogenase activities. When cells were incubated in -ketoglutarate-containing medium, without added H₂ but in the presence of acetylene (to block H₂ evolution from nitrogenase) significant hydrogenase activity was still observed. Complete inhibition of nitrogenase-dependent H₂ evolution by acetylene was verified by the use of a Hup^- mutant. Hydrogen is therefore not required to induce hydrogenase. The presence of 10% acetylene inhibited derepression of hydrogenase. Constitutive (Hup^c) mutants were isolated which contained up to 9 times the level of hydrogenase acitivity than the wild type in nitrogenase induction medium. These mutants did not have greater nitrogenase activities than the wild type.This is contribution number 1254 from the Department of Biology and the McCollum-Pratt Institute Abbreviations: -Ketoglutarate-containing medium (LOKG) and pre-adaptation medium (SRM) as described in Materials and methods     

CMP-N-acetylneuraminic acid hydrolase, an ectoenzyme distributed unevenly over the hepatocyte surface.

The regional localization of CMP-N-acetylneuramic acid hydrolase at the hepatocyte surface was studied by using plasma membranes and hepatocytes isolated from rat liver. 1. By homogenization of the rat liver plasma membrane preparations and subsequent discontinuous sucrose gradient centrifugation, one light and two heavy membrane fractions were obtained. The origin of these three subfractions is discussed based on the specific activities in the three fractions of 5'-nucleotidase, alakaline phosphatase and Mg2+-ATPase and on electron microscopic examination of the fractions. Evidence is given suggesting that the light fraction is derived from the bile canalicular surface of the plasma membrane, and that the heavy fractions are derived predominantly from the sinusoidal and lateral surfaces of the liver cell membrane. CMP-AcNeu hydrolase was present at highest specific activity in one of the heavy subfractions. Therefore it is concluded that CMP-AcNeu hdyrolase is located preferentially in the sinusoidal and/or lateral plasma membrane parts of the liver cell. 2. Experiments with intact and disintegrated hepatocytes isolated from rat liver indicated that CMP-AcNeu hydrolase is located at the surface of the cell membrane, with its functional group directed to the outside.     

Rhizobium japonicum mutants that are hypersensitive to repression of H2 uptake by oxygen.

The synthesis of an H2 oxidation system in free-living Rhizobium japonicum wild-type strain SR is repressed by oxygen. Maximal H2 uptake rates were obtained in strain SR after derepression in 11 microM or less dissolved oxygen. Oxygen levels above 45 microM completely repressed H2 uptake in strain SR. Five R. japonicum mutant strains that are hypersensitive to repression or H2 oxidation by oxygen were derived from strain SR. The mutants were obtained by screening H2 uptake-negative mutants that retained the ability to oxidize H2 as bacteroids from soybean nodules. As bacteroids, the five mutant strains were capable of H2 oxidation rates comparable to that of the wild type. The mutants did not take up H2 when derepressed in 22 microM dissolved oxygen, whereas strain SR had substantial activity at this oxygen concentration. The O2 repression of H2 uptake in both the wild-type and two mutant strains, SR174 and SR200, was rapid and was similar to the effect of inhibiting synthesis of H2 uptake system components with rifampin. None of the mutant strains was able to oxidize H2 when the artificial electron acceptors methylene blue or phenazine methosulfate were provided. The mutant strains were not sensitive to killing by oxygen, they took up O2 at rates similar to strain SR, and they did not produce an H2 uptake system that was oxygen labile. Cyclic AMP levels were comparable in strain SR and the five mutant strains after subjection of the cultures to the derepression conditions.     

A spin labeling study of the effects of inorganic ions and pH on the conformation of spectrin

The structure of spectrin from human erythrocytes has been investigated by the EPR technique measuring the mobility of the protein spin label, 4-maleimido-2,2,6,6-tetramethylpiperidinooxyl. Conformational changes in the protein induced by variation of the concentrations of NaCl, Na2SO4, KCl, CaCl2 and MgCl2 and of pH have been studied. It could be demonstrated that both Ca2+ and Mg2+ give rise to structural changes by binding to specific sites, whereas the monovalent cations (K+, Na+) seem to act via ionic strength. A model is used to correlate the spin label mobility with the radius of the protein. In the Ca2+- and Mg2+-binding experiments, the decrease in the spin label mobility has been interpreted on the basis of the theory of multiple chemical equilibria. These experiments have been compared with EPR spectra measured at different pH values. The results support the model in that binding of H+, Ca2+ or Mg2+ reduces the charges located on the protein surface: the 'discharging' reduces the repulsive forces on the surface of the molecule and consequently, the protein contracts in discrete steps.     

Influence of subcellular fractions of mammalian testes on the mutagenic activity of nitrofurans toward Escherichia coli; presence of a co-mutagen-like factor.

The activation of nitrofurans to mutagenic intermediates by testicular tissue was investigated. AF-2 and nitrofurazone were tested in a microsomal suspension assay with strain E. coli K-12 343/113 as indicator and subcellular fractions from rabbit testes. Different mutation patterns were observed in the presence or absence of testicular homogenate, indicating the presence of different mutagenic intermediates. The frequency of arg+ reversion increased proportionally to the homogenate concentration suggesting that the nitrofurans were activated by testicular components to intermediates that induced base-pair substitutions. Other experiments showed that a component of low molecular weight, present in the soluble fraction of homogenates of testes from rabbits, rats and monkeys, was responsible for the increased mutation frequency. It is concluded that this "co-mutagen-like" factor either alters the metabolism of nitrofurans in E. coli and/or promotes the formation of base-pair substitution-type mutations. This direct interaction between a nonenzymic component of mammalian testes and the mutation induction/expression process in E. coli suggests the role of co-mutagen-like factors in the sensitivity of testes to nitrofurans.     

Primary structure differences of human surfactant-associated proteins isolated from normal and proteinosis lung.

The primary structures of human pulmonary surfactant-associated proteins SP-A, SP-B and SP-C isolated from lung lavage of patients with alveolar proteinosis exhibit significant differences from lung surfactant proteins isolated from lungs of healthy individuals. In contrast to SP-A from normal lungs, proteinosis SP-A was shown by SDS gel electrophoresis to contain large amounts of unreducibly cross-linked beta chains. Specific primary structure modifications of SP-C and SP-B proteins were established by direct molecular weight and structural analysis, using [252Cf]plasma desorption mass spectrometry (PD/MS) as the principal method. In comparison to normal lung surfactant SP-B, proteinosis SP-B showed a significantly increased molecular weight by approx. 500 Da for the unreduced protein dimer. SP-C proteins from normal lungs were identified to possess a bis-cysteinyl-5,6-(thioester)palmitoylated structure, and to contain a frayed N-terminus resulting in two sequences of 34 and 35 amino acid residues. In contrast, SP-C from proteinosis patients was modified by (i) partial or even complete removal of palmitate residues and (ii) additional N-terminal proteolytic degradation. These results indicate the presence of pathophysiological structure modifications, which are likely to occur in the alveolar space, and may lead to a reduced surfactant function.