Development of self-reactive germinal center B cells and plasma cells in autoimmune Fc gammaRIIB-deficient mice

The leukemogenic effects of Myc drive recurrent trisomy in a mouse model of acute myeloid leukemia.

Abnormalities in expression levels of the IgG inhibitory Fc gamma receptor IIB (FcγRIIB) are associated with the development of immunoglobulin (Ig) G serum autoantibodies and systemic autoimmunity in mice and humans. We used Ig gene cloning from single isolated B cells to examine the checkpoints that regulate development of autoreactive germinal center (GC) B cells and plasma cells in FcγRIIB-deficient mice. We found that loss of FcγRIIB was associated with an increase in poly- and autoreactive IgG⁺ GC B cells, including hallmark anti-nuclear antibody–expressing cells that possess characteristic Ig gene features and cells producing kidney-reactive autoantibodies. In the absence of FcγRIIB, autoreactive B cells actively participated in GC reactions and somatic mutations contributed to the generation of highly autoreactive IgG antibodies. In contrast, the frequency of autoreactive IgG⁺ B cells was much lower in spleen and bone marrow plasma cells, suggesting the existence of an FcγRIIB-independent checkpoint for autoreactivity between the GC and the plasma cell compartment.

Site blocking and CO/sulfur site exchange processes on stepped Pt surfaces

The influence of preadsorbed sulfur on the adsorption of CO on Pt(355) and Pt(322) is investigated systematically for sulfur precoverages between 0.02 and 0.30 ML by in situ x-ray photoelectron spectroscopy of the C 1s and S 2p core levels. The two surfaces have the same nominal terrace width of 5 atomic rows, but different step orientation. For both, at low temperatures (130 and 150 K), S preferentially adsorbs at the steps and passivates them for CO adsorption. The required S precoverage is significantly smaller for Pt(322), because of the lower number of steps as a result of the S-induced double-step formation. Upon heating, population of step sites with CO occurs due to irreversible CO/S site exchange. At low S precoverages, the characteristic transition temperatures of 165 K for Pt(355) and 245 K for Pt(322) are significantly different, indicating a higher activation barrier for the CO/S site exchange process for Pt(322). For medium to high S precoverages the transition temperature stays unchanged for Pt(322) but increases for Pt(355). The observed behavior is attributed to a kinetic passivation of the steps by sulfur at low temperatures, which is lifted upon heating.

Autonomously replicating macronuclear DNA pieces are the physical basis of genetic coassortment groups in Tetrahymena thermophila

The macronucleus of the ciliate Tetrahymena thermophila contains a fragmented somatic genome consisting of several hundred identifiable chromosome pieces. These pieces are generated by site-specific fragmentation of the germline chromosomes and most of them are represented at an average of 45 copies per macronucleus. In the course of successive divisions of an initially heterozygous macronucleus, the random distribution of alleles of loci carried on these copies eventually generates macronuclei that are pure for one allele or the other. This phenomenon is called phenotypic assortment. We have previously reported the existence of loci that assort together (coassort) and hypothesized that these loci reside on the same macronuclear piece. The work reported here provides new, rigorous genetic support for the hypothesis that macronuclear autonomously replicating chromosome pieces are the physical basis of coassortment groups. Thus, coassortment allows the mapping of the somatic genome by purely genetic means. The data also strongly suggest that the random distribution of alleles in the Tetrahymena macronucleus is due to the random distribution of the MAC chromosome pieces that carry them.

Tetrahymena micronuclear genome mapping. a high-resolution meiotic map of chromosome 1l

The ciliate Tetrahymena thermophila is a useful model organism that combines diverse experimental advantages with powerful capabilities for genetic manipulation. The genetics of Tetrahymena are especially rich among eukaryotic cells, because it possesses two distinct but related nuclear genomes within one cytoplasm, contained separately in the micronucleus (MIC) and the macronucleus (MAC). In an effort to advance fulfillment of Tetrahymena's potential as a genetic system, we are mapping both genomes and investigating the correspondence between them. With the latter goal especially in mind, we report here a high-resolution meiotic linkage map of the left arm of chromosome 1, one of Tetrahymena's five chromosomes. The map consists of 40 markers, with an average spacing of 2.3 cM in the Haldane function and a total length of 88.6 cM. This study represents the first mapping of any large region of the Tetrahymena genome that has been done at this level of detail. Results of a parallel mapping effort in the macronucleus, and the correspondence between the two genomes, can be found in this issue as a companion to this article.

Tetrahymena macronuclear genome mapping: colinearity Of macronuclear coassortment groups and the micronuclear map on chromosome 1l

The genetics of the ciliate Tetrahymena thermophila are richer than for most other eukaryotic cells, because Tetrahymena possesses two genomes: a germline (micronuclear) genome that follows a Mendelian model of genetic transmission and a somatic (macronuclear) genome, derived from the micronuclear genome by fragmentation, which follows a different genetic transmission model called phenotypic assortment. While genetic markers in the micronucleus fall into classical linkage groups under meiotic recombination and segregation, the same markers in the macronucleus fall into coassortment groups (CAGs) under phenotypic assortment by the random distribution of MAC chromosome pieces. We set out to determine whether genomic mapping in the macronucleus by genetic means is feasible. To investigate the relationship between the micronuclear map and coassortment groups, we systematically placed into CAGs all of the markers lying on chromosome 1L that are also found in the macronucleus. Sixteen CAGs were identified, 7 of which contain at least two loci. We have concluded that CAGs represent a fundamental genetic feature of the MAC. The MIC and MAC maps on 1L are colinear; that is, CAGs consist exclusively of markers that map to a continuous segment in a given region of the micronuclear map, with no intervening markers from other CAGs. These findings provide a solid foundation for exploiting the MAC chromosome pieces to build a physical map of the Tetrahymena genome.

A small protein (Ags1p) and the Pho80p-Pho85p kinase complex contribute to aminoglycoside antibiotic resistance of the yeast Saccharomyces cerevisiae

We identified the AGS1 and AGS3 genes by their ability to partially complement an ags mutant (RC1707) which is supersensitive to various aminoglycoside antibiotics (J. F. Ernst and R. K. Chan, J. Bacteriol. 163:8-14, 1985). AGS1 is located in proximity to the centromere of chromosome III and encodes a small protein of 88 amino acids. The size of the AGS1 transcript, which in wild-type cells is 1 kb, is reduced to 0.75 kb in mutant RC1707. Disruption of AGS1 rendered strains supersensitive to hygromycin B and increased their resistance to vanadate. In addition, ags1delta strains underglycosylated invertase but had normal carboxypeptidase Y glycosylation, suggesting that Ags1p is required for the elaboration of outer N-glycosyl chains. AGS3 was found to be identical to PHO80 (TUP7), which encodes a cyclin activating the Pho85p protein kinase. Deletion of either PHO80 or PHO85 led to aminoglycoside supersensitivity; pho80delta ags1delta strains showed an enhanced-sensitivity phenotype compared to single mutants. pho80 and pho85 mutants were rendered resistant by deletion of PHO4, indicating that activation of the Pho4p transcription factor is required for increased aminoglycoside sensitivity. Thus, both the Pho80p-Pho85p kinase complex (by Pho4p phosphorylation) and a novel component of the N glycosylation pathway contribute to basal levels of aminoglycoside resistance in Saccharomyces cerevisiae.

Analysis of sequence and pathogenic properties of two variants of encephalomyocarditis virus differing in a single amino acid in VP1

The encephalomyocarditis (EMC) virus-induced diabetes-like syndrome in mouse inbred strains was used as a model to study the insulin-dependent diabetes mellitus (IDDM). Our investigations were performed with two EMC virus variants, PV2 and PV7. After infection of SJL mice with 10(5) PFU of PV2 about 70% of the animals developed a diabetes-like syndrome, whereas the PV7 infected mice appeared healthy. Histological examination and in situ experiments revealed that the islets of Langerhans are a main target of PV2, whereas PV7 infection leads to only modest changes of the islets. Sequence analysis of both variants revealed one amino acid exchange within the capsid protein VP1. Hence, we describe the first diabetogenic and non-diabetogenic EMCV variants differing in only one single amino acid.