One-carbon metabolism in lectin-activated human lymphocytes

Serine is an essential amino acid for the lectin-mediated transformation of human peripheral blood lymphocytes due to the inability of this cell to synthesize sufficient quantities via either the phosphorylated pathway or by reversal of the serine hydroxymethyltransferase reaction to meet the metabolic demands. The level of intracellular serine is tightly regulated, and the culture medium concentration for optimum cellular transformation falls within a relatively narrow range. The three-carbon atom of serine is the major source of one-carbon units required for purine and pyrimidine nucleotide biosynthesis, but the key effect of both serine deprivation and of high medium serine levels would appear to be on protein synthesis. Although an alternative source of one-carbon units, as provided by high levels of formate in the culture medium, can partially reverse the effects of serine deprivation, the only other demonstrable source of one-carbon units, tryptophan, requires serine for its incorporation and subsequent metabolism. Methionine is also essential for lymphocyte transformation and is involved in the synthesis of a small amount of phosphatidylcholine, although most of this phospholipid is provided by choline and lysophosphatidylcholine from the serum-supplemented culture medium.     

Alpha 1-antitrypsin and serum albumin mRNA accumulation in normal, acute phase and ZZ human liver

Alpha 1-Antitrypsin and albumin mRNA levels of 4 human livers were assessed using a newly sequenced cDNA clone of the carboxyterminal third of alpha 1-antitrypsin and a previously cloned albumin cDNA sequence. The relative concentration of alpha 1-antitrypsin mRNA was the same in poly(A)-containing RNA isolated from acute phase (MM) and alpha1-antitrypsin deficient (ZZ) individuals. In the acute phase liver relative to the normal (MM) liver, total RNA extracts showed a marked decrease in albumin mRNA concentration but no increase in alpha 1-antitrypsin mRNA. The ZZ liver showed decreased total and poly(A)-containing RNA content but the same proportion of alpha 1-antitrypsin to albumin mRNA as in the normal (MM) liver. This supports other evidence that ZZ alpha 1-antitrypsin deficiency is due to a defect in polypeptide processing (secretion) rather than a deficiency in mRNA accumulation.     

Arthrin: a new actin-like protein in insect flight muscle

There are one or more proteins of 50,000 to 60,000 Mr in the thin filaments of insect flight muscle. A protein of 55,000 Mr has been isolated from insect fibrillar flight muscle and called arthrin. Despite its higher molecular weight, arthrin is in many ways like actin. The amino acid composition of arthrin was similar to that of actin. There were similarities in the peptides produced by digesting the denatured proteins and mild digestion of polymerized proteins cleaved similar-sized fragments from arthrin and actin. Polymerized arthrin activated the Mg2+ ATPase of myosin to the same extent as actin and the ATPase was regulated by rabbit or Lethocerus troponin and tropomyosin. Arthrin did not itself act as troponin-T. Electron microscopy of negatively stained specimens showed that arthrin and actin filaments were similar in structure and that arthrin could be decorated by rabbit subfragment-1 to form normal-looking arrowheads. Arthrin formed paracrystals at an optimum concentration of MgCl2 (25 mM) that was somewhat lower than the optimum for actin paracrystals. Optical diffraction showed that the structure of the paracrystals was similar to those formed from actin. The mass of arthrin and actin filaments relative to phage fd was measured by scanning transmission electron microscopy; the relative mass of arthrin and actin was 1.33, in agreement with molecular weight estimations. Therefore arthrin has the properties of a heavy form of actin. The proportion of actin, arthrin and troponin-T in Lethocerus myofibrils was six moles of actin to one mole of arthrin and one mole of troponin-T. The function of arthrin is not known.     

Isolation and characterization of an alphoid centromeric repeat family from the human Y chromosome

A collection of human Y-derived cosmid clones was screened with a plasmid insert containing a member of the human X chromosome alphoid repeat family, DXZ1. Two positive cosmids were isolated and the repeats they contained were investigated by Southern blotting, in situ hybridization and sequence analysis. On hybridization to human genomic DNAs, the expected cross-hybridization characteristic of all alphoid sequences was seen and, in addition, a 5500 base EcoRI fragment was found to be characteristic of a Y-specific alphoid repeat. Dosage experiments demonstrated that there are about 100 copies of this 5500 base EcoRI alphoid fragment on the Y chromosome. Studies utilizing DNA from human-mouse hybrids containing only portions of the Y chromosome and in situ hybridizations to chromosome spreads demonstrated the Y centromeric localization of the 5500 base repeat. Cross-hybridization to autosomes 13, 14 and 15 was also seen; however, these chromosomes lacked detectable copies of the 5500 base EcoRI repeat sequence arrangement. Sequence analysis of portions of the Y repeat and portions of the DXZ1 repeat demonstrated about 70% homology to each other and of each to the human consensus alphoid sequence. The 5500 base EcoRI fragment was not seen in gorilla, orangutan or chimpanzee male DNA.     

5S and 5.8S ribosomal RNA evolution in the suborder tetrahymenina (Ciliophora: Hymenostomatida)

Summary The nucleotide sequences of the 5S and 5.8S rRNAs of eight strains of tetrahymenine ciliates have been determined. The sequences indicate a clear distinction betweenTetrahymena paravorax and its suggested conspecificT. vorax, but leave the taxonomic distinction betweenT. vorax andT. leucophrys in doubt. The rRNA sequences of sixTetrahymena species and of three other species of the suborder Tetrahymenina have been used to deduce evolutionary schemes in which ancestral rRNA sequences and changes are proposed. These schemes suggest the predominant acceptance of GA and CT transitions in the 5S rDNA during the evolution of the suborder.     

Cell-cell interaction can influence drug-induced differentiation of murine embryonal carcinoma cells

When cultured in the presence of either retinoic acid (RA) or dimethyl sulfoxide (DMSO), aggregates of the P19 line of mouse embryonal carcinoma (EC) cells differentiate and the spectrum of cell types formed depends on the drug dose. It is shown here the EC cells rapidly lose their colony-forming ability when cultured as aggregates in the presence of DMSO. This loss of plating efficiency (PE) also occurs rapidly following RA treatment. Loss of PE has been used as a quantitative procedure for assessing the rate of drug-induced differentiation. The relationship between drug dose and loss of PE is much steeper for DMSO than for RA, suggesting that these two drugs affect different stages of the differentiation decision-making apparatus. Mutant EC cell lines (D3 and RAC65) do not differentiate in the presence of drug-inducers (DMSO and RA, respectively). Neither differentiation-deficient mutant has an altered ability to form gap junctions. When D3 and P19 cells were mixed within the same DMSO-treated aggregates, the D3 cells remained undifferentiated and the P19 cells differentiated much less efficiently than if they were cultured in the absence of the D3 cells. When RAC65 and P19 cells were mixed in RA-treated aggregates, each cell responded to the drug as though the other were absent. Thus RA behaves as a cell-autonomous inducer of differentiation, whereas DMSO-induced differentiation seems to be mediated by interactions between neighboring cells.     

A transient inward current elicited by hyperpolarization during serotonin activation in Xenopus oocytes

Activation of serotonin, glutamate or muscarinic receptors, incorporated into the membrane of Xenopus oocytes following injection of messenger RNA from rat brain, caused the development of a transient inward (Tin) current when the membrane was hyperpolarized. A detailed study was made of the Tin current induced during serotonin activation. The current is due principally to efflux of chloride ions, and is presumably activated by an influx of calcium ions, because it was blocked by removal of calcium from the bathing medium, by addition of manganese, cobalt or lanthanum, or by intracellular injection of EGTA. During application of serotonin, the amplitude of the Tin current increased slowly, and after washing it persisted for longer than the direct serotonin-induced current. The amplitude of the Tin current was sensitive to temperature and pH, and was abolished at pH 6.5 or by cooling to 12 degrees C. The Tin current may be of importance in regulating the excitability of neurons in the central nervous system.     

Studies of heterogeneous mitochondrial populations in a mouse cell line: the effects of selection for or against mitochondrial genomes that confer chloramphenicol resistance

A single nucleotide substitution in a highly conserved region of the mitochondrial genome of a mouse cell line confers both chloramphenicol resistance and an alteration to the recognition site for the endonuclease Eco RV. This has enabled a detailed study on the effects of selection on a mitochondrial population comprising initially both chloramphenicol-resistant and chloramphenicol-sensitive mitochondrial genomes. The mutation confers advantage to cells grown in the presence of chloramphenicol, but is apparently deleterious in its absence. Selection at the cellular level is sufficient to explain the results observed. Fixation, which results in cells having mitochondria of only a single type, is slow. It is probable, therefore, that mammalian oocyte mitochondria are derived from only a small number of progenitors. This would allow fixation of new mutations and explain the observed uniformity in mitochondrial genomes of the individual in the presence of extensive variation between different members of the population.