An intramolecular recombination mechanism for the formation of the rRNA gene palindrome of Tetrahymena thermophila.

Large palindromic DNAs are found in a wide variety of eukaryotic cells. In Tetrahymena thermophila, a large palindrome is formed from a single rRNA gene (rDNA) during nuclear differentiation. We present evidence that a key step in the formation of the rDNA palindrome of T. thermophila involves homologous intramolecular recombination. Heteroduplex micronuclear rDNA molecules were constructed in vitro and microinjected into developing macronuclei, where they formed palindromes. Analysis of the resulting palindromes indicated that both strands of the microinjected rDNA are used to form the same palindrome. This study, together with a previous study (L. F. Yasuda and M.-C. Yao, Cell 67:505-516, 1991), is the first to define a molecular pathway of palindrome formation. The process is initiated by chromosome breakage at sites flanking the micronuclear rDNA. An intramolecular recombination reaction, guided by a pair of short inverted repeats located at the 5' end of the excised rDNA, covalently joins the two strands of micronuclear rDNA in a giant hairpin molecule. Bidirectional DNA replication converts the giant hairpin molecule to a palindrome. We suggest that the general features of this pathway are applicable to palindrome formation in other cell types.     

Amphiphysin II (SH3P9; BIN1), a Member of the Amphiphysin/Rvs Family, Is Concentrated in the Cortical Cytomatrix of Axon Initial Segments and Nodes of Ranvier in Brain and around T Tubules in Skeletal Muscle

Amphiphysin (amphiphysin I), a dominant autoantigen in paraneoplastic Stiff-man syndrome, is a neuronal protein highly concentrated in nerve terminals, where it has a putative role in endocytosis. The yeast homologue of amphiphysin, Rvs167, has pleiotropic functions, including a role in endocytosis and in actin dynamics, suggesting that amphiphysin may also be implicated in the function of the presynaptic actin cytoskeleton. We report here the characterization of a second mammalian amphiphysin gene, amphiphysin II (SH3P9; BIN1), which encodes products primarily expressed in skeletal muscle and brain, as differentially spliced isoforms. In skeletal muscle, amphiphysin II is concentrated around T tubules, while in brain it is concentrated in the cytomatrix beneath the plasmamembrane of axon initial segments and nodes of Ranvier. In both these locations, amphiphysin II is colocalized with splice variants of ankyrin3 (ankyrin_G), a component of the actin cytomatrix. In the same regions, the presence of clathrin has been reported. These findings support the hypothesis that, even in mammalian cells, amphiphysin/Rvs family members have a role both in endocytosis and in actin function and suggest that distinct amphiphysin isoforms contribute to define distinct domains of the cortical cytoplasm. Since amphiphysin II (BIN1) was reported to interact with Myc, it may also be implicated in a signaling pathway linking the cortical cytoplasm to nuclear function.     

Correlation between serum ionised calcium and serum albumin concentrations in two hospital populations.

One quarter of 172 patients from two hospitals with no obvious disturbances of calcium homeostasis and with total serum calcium concentrations that were normal after adjustment for albumin concentration had low serum ionised calcium concentrations. The low values were not due to changes in pH but were associated with hypoalbuminaemia. Significant positive regressions of ionised calcium on albumin concentration were observed in patients from both hospitals and also in 48 healthy laboratory staff. Because the regressions did not differ between patients and healthy subjects the low ionised calcium values associated with hypoalbuminaemia are unlikely to have been of pathological importance. These findings indicate that interpreting serum ionised calcium concentrations in patients with a reduced serum albumin concentration on the basis of a reference range determined in subjects with a normal serum albumin concentration may be clinically misleading.     

The covalent structure of cartilage collagen. Amino acid sequence of residues 552–661 of bovine α1(II) chains

The covalent structure of the first 111 residues from the N-terminus of peptide α1(II)-CB10 from bovine nasal-cartilage collagen is presented. This region comprises residues 552–661 of the α1(II) chain. The sequence was determined by automated Edman degradation of peptide α1(II)-CB10 and of peptides produced by cleavage with trypsin and hydroxylamine. Comparison of this region of the α1(II) chain with the homologous segment of the α1(I) chain indicated a homology level of 85%, slightly higher than that of 81% reported for the N-terminal region of the α1(II) chain (Butler, Miller & Finch (1976) Biochemistry 15, 3000–3006). The occurrence of two residues of glycosylated hydroxylysine was established at positions 564 and 603, the first present exclusively as galactosylhydroxylysine and the latter as a mixture of galactosylhydroxylysine and glucosylgalactosylhydroxylysine. Also, two residues at positions 648 and 657 were tentatively identified as glycosylated hydroxylysines. The amino acid sequences adjacent to the hydroxylysine residues so far identified in the α1(II) chain were compared with the homologous regions of the α1(I) and α2 chains, but no obvious prerequisite for hydroxylation could be seen. From comparison with the homologous sequence of the α1(I) chain, it appears that the α1(II)-chain sequence presented here contains three more amino acids than that reported for the α1(I) chain. This triplet would be interposed between residues 63 and 64 of the reported sequence of peptide α1(I)-CB7 from calf skin collagen. Data on the purification of the subpeptides and their amino acid compositions have been deposited as Supplementary Publication SUP 50087 (7 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1978) 169, 5.     

Use of phospholipase A to compare phospholipid organization in synaptic membranes,myelin, and liposomes

Summary The pattern of fatty acid release from rat synaptic membranes in the presence of phospholipase A₂ (Vipera russelli) was compared to that from liposomes comprised of phospholipids. Phospholipase A₂ more readily attacked myelin and synaptic membranes than liposomes prepared from total phospholipids derived from myelin. Although hydrolysis of liposomal phospholipids occurred in the absence of added calcium, the presence of 2mm CaCl₂ or 2% bovine serum albumin significantly enhanced the phospholipase attack of liposomes, but not synaptic membranes or myelin. Phospholipase exhibited a marked preference for phospholipids containing docosahexaenoic acid (226) in the synaptic membranes, while with liposomes the pattern of released fatty acid reflected the fatty acid composition in the two-position of the phospholipids. Although either calcium or albumin markedly increased the phospholipase hydrolysis of liposomes, neither affected the hydrolysis of synaptic membranes or the pattern of fatty acid release from liposomes. It was concluded that the nonlipid constituents, particularly the proteins, of biomembranes were responsible for the organization of the phospholipids and accounted for the observed differences between liposomes and synaptic membranes with respect to enzymic accessibility.     

The electronic structure of Fe2+ in reaction centers from Rhodopseudomonas sphaeroides. I. Static magnetization measurements.

We have measured the static magnetization of unreduced and reduced reaction centers that vary in their quinone content. Measurements were performed in the temperature range 0.7 degrees K less than T less than 200 degrees K and magnetic fields of up to 10 kG. The electronic g-value, crystal field parameters D, E, and the exchange interaction, J, between the quinone spin and Fe2+ were determined using the spin Hamiltonian formalism. The effective moment mu eff/Fe2+ of both reduced and unreduced samples were determined to be 5.35 +/- 0.15 Bohr magnetons. This shows, in agreement with previous findings, that Fe2+ does not change its valence state when the reaction centers are reduced. Typical values of D congruent to +5 cm-1 and E/D congruent to 0.27 are consistent with Fe being in an octahedral environment with rhombic distortion. The values of D and E were approximately the same for reaction centers having one and two quinones. These findings imply that quinone is most likely not a ligand of Fe. The Fe2+ and the spin on the quinone in reduced reaction centers were found to be coupled with an exchange interaction 0 less than /J/ less than 1 cm-1. The validity of the spin Hamiltonian was checked by using an orbital Hamiltonian to calculate energy levels of the 25 states of the S = 2, L = 2 manifold and comparing the magnetization of the lowest five states with those obtained from the spin Hamiltonian. Using the orbital Hamiltonian, we calculated the position of the first excited quintet state to be 340 cm-1 above the ground state quintet. This is in good agreement with the temperature dependence of the quadrupole splitting as determined by Mossbauer spectroscopy.     

Structure of transcriptionally-active chromatin subunits.

Rat liver chromatin is organized into regions of DNA which differ in degree of susceptibility to attack by the endonucleases DNase I and DNase II. The most nuclease-sensitive portion of chromatin DNA is enriched in transcribed sequences. This fraction may be separated from the bulk of chromatin by virtue of its solubility in solutions containing 2 mM MgCl2. Both transcribed and nontranscribed regions of chromatin are organized into repeating units of DNA and histone, which appear as 100 A beads in the electron microscope. The length of DNA in the repeat unit is the same for these two classes of chromatin (198 +/- 6 base pairs in rat liver); however, the subunits of active, Mg++-soluble chromatin differ from the nucleosomes of inactive regions of chromatin in several respects. Active subunits are enriched in nascent RNA and nonhistone protein and exhibit higher sedimentation values than the corresponding subunits of inactive chromatin.