Chemotaxis of Rhizobium meliloti to the plant flavone luteolin requires functional nodulation genes.

Luteolin is a phenolic compound from plants that acts as a potent and specific inducer of nodABC gene expression in Rhizobium meliloti. We have found that R. meliloti RCR2011 exhibits positive chemotaxis towards luteolin. A maximum chemotactic response was observed at 10(-8) M. Two closely related flavonoids, naringenin and apigenin, were not chemoattractants. The presence of naringenin but not apigenin abolished chemotaxis of R. meliloti towards luteolin. A large deletion in the nif-nod region of the symbiotic megaplasmid eliminated all chemotactic response to luteolin but did not affect general chemotaxis, as indicated by swarm size on semisoft agar plates and chemotaxis towards proline in capillary tubes. Transposon Tn5 mutations in nodD, nodA, or nodC selectively abolished the chemotactic response of R. meliloti to luteolin. Agrobacterium tumefaciens GMI9050, a derivative of the C58 wild type lacking a Ti plasmid, responded chemotactically to 10(-8) M luteolin. The introduction of a 290-kilobase nif-nod-containing sequence of DNA from R. meliloti into A. tumefaciens GMI9050 enabled the recipient to respond to luteolin at concentrations peaking at 10(-6) M as well as at concentrations peaking at 10(-8) M. The response of A. tumefaciens GMI9050 to luteolin was also abolished by the presence of naringenin.     

Thyroid function and trisomy 21. TSH increase and rT3 deficiency

An excess of thyrotropin (TSH) with normal levels of tetraiodothyronine (T4) and of 3,5,3'-triiodothyronine (T3) was confirmed in the serum of 78 trisomy 21 children. A severe deficiency of 3,3',5'-triiodo-thyronine (rT3 or reverse T3) was observed and the decrease of the rT3/TSH ratio was highly significant. These new facts suggest that the rT3 deficiency plays a peculiar role in trisomy 21 (maybe through the regulation of one or few steps of monocarbons' metabolism). A systematic control of thyroid function (including the patient's rT3 level) is mandatory for the follow-up of every trisomy 21 patient.     

Multiple binding of bilirubin to human serum albumin and cobinding with laurate

Numerical analysis of multiple binding of two ligands to one carrier has been accomplished, using the principle of several sets of acceptable binding constants, with bilirubin-laurate-albumin as an example. Binding of bilirubin to defatted human serum albumin was investigated by a spectroscopic method, based upon a difference of light absorption spectrum for free and bound bilirubin. The observations were supplemented with previous data from an independent technique, measurement of oxidation rates of free bilirubin with hydrogen peroxide and peroxidase. A continuous isotherm was obtained, showing binding of at least 4 mol bilirubin per mole albumin with the following stoichiometric binding constants, 1.11 X 10(8), 1.7 X 10(7), 8 X 10(5), and 4 X 10(4) M-1 at pH 8.2, ionic strength 0.15 M, 25 degrees C. The binding is anticooperative at all steps. A saturation level was not reached. Cobinding of bilirubin and laurate was studied, with up to 2 mol of each ligand per mole albumin, using the peroxidase method for determination of free equilibrium concentrations of bilirubin, and a dialysis rate technique for free laurate. The findings could be described in terms of a stoichiometric model. Heterotropic cooperativity was present among the first bilirubin and the first and second laurate molecules. More than two molecules of either ligand can be bound at the same time.     

Analysis of the aromatic 1H NMR spectrum of chicken plasminogen kringle 4

The intact kringle 4 domain of chicken plasminogen has been characterized by 1H NMR spectroscopy at 300 and 620 MHz in both the presence and absence of epsilon-aminocaproic acid, an antifibrinolytic drug. The study focuses on the aromatic resonances. Comparisons with spectra from human, porcine and bovine kringle 4 homologs indicates a strict conservancy of conformation, reflecting the underlying primary sequence homology, and leads to an unambiguous assignment of all the aromatic resonances, including those of Phe15 and His40 which are unique to the chicken domain. Conclusive evidence is found that the Tyr9 ring fluctuates between two states, one in which it flips fast and other in which it is severely hindered. Similarly, the Tyr64 side chain finds itself in a structurally constrained locus. The Trp62, Tyr64, and Trp72 aromatic resonances are most sensitive to ligand presence, supporting a previously reported model of the kringle 4 lysine-binding site. His40, Phe41, and Tyr74 are also perturbed by ligand indicating proximity to the site. In contrast, the Phe15 aromatic spectrum indicates a rather mobile phenyl ring which is insensitive to ligand presence, thus confirming the lesser importance of the corresponding segment within the first kringle loop in determining kringle structure and/or function.     

Amino acid sequence of the N-terminus and selected tryptic peptides of the active subunit of human plasma carboxypeptidase N: comparison with other carboxypeptidases

Human plasma carboxypeptidase N was purified to homogeneity and its active and inactive subunits were separated. By introducing a novel technique, both forms of the active subunit (Mr = 55,000 and Mr = 48,000) were isolated. N-terminal sequencing of the active subunit of human carboxypeptidase N revealed significant homology with the N-terminal sequence of bovine carboxypeptidase H (43% identity) and to a lesser extent with carboxypeptidase A (29% identity) or carboxypeptidase B (18% identity). The active subunit of carboxypeptidase N was hydrolyzed with trypsin and 4 of the tryptic peptides were isolated by HPLC and sequenced. The sequences of the four peptides were homologous (39-64% identity) with regions of carboxypeptidase H corresponding to the middle (residues 148-175) and C-terminal portion (residues 321-408). These regions had essentially no homology with carboxypeptidase A or B. These data indicate that carboxypeptidase H and the active subunit of carboxypeptidase N may have diverged from a common ancestral gene.     

Characterization, localization, and biosynthesis of acetylcholinesterase in K 562 cells

K 562 cell acetylcholinesterase (AChE), identifiable by active site labeling with radioactive diisopropylfluorophosphate (DFP), showed a Mr around 55,000 in both a crude lysate and a purified sample. The K 562 AChE was reactive with one polyclonal and two monoclonal antibodies produced against human erythrocyte AChE. Subcellular localization, investigated by assay on cell fractions, showed that AChE is membrane bound and that it is located on the cell surface as well as on microsomal and Golgi membranes. Biosynthesis of new enzyme molecules, after inactivation of the constitutive AChE with the irreversible inhibitor DFP, allowed us to follow the kinetics of reappearance in the intracellular compartment and at the cell surface (4 and 8 h, respectively).     

Periodic cleavage of poly(dA) by oligothymidylates covalently linked to the 1,10-phenanthroline-copper complex

1,10-Phenanthroline (OP) was covalently attached to the 3'-terminus of two oligothymidylates via different linkers [abbreviated as T8-(OP) and T6-(OP)]. In the presence of Cu2+ and 3-mercaptopropionic acid (MPA), these reagents induce a hybridization-dependent cleavage of poly(dA) and of a 27 nucleotide long oligodeoxynucleotide containing an A8 sequence. The principal cleavage sites on the 27-mer span four residues located near the 3'-terminal phosphate group of T8-(OP). When poly(dA) was degraded by T6-(OP) and T8-(OP), a series of bands were obtained corresponding to a repeat unit of six and eight nucleotides, respectively. This periodicity reflects the cooperative binding of oligothymidylate-OP to the polynucleotide matrix and the localized nicking sites.