A Test of Neutrality Based on Interlocus Associations

The evolutionary processes governing variability within genomic regions of low recombination have been the focus of many studies. Here, I investigate the statistical properties of a measure of intrlocus genetic associations under the assumption that mutations are selectively neutral and sites are completely linked. This measure, denoted Z(nS), is based on the squared correlation of allelic identity at pairs of polymorphic sites. Upper bounds for Z(nS) are determined by simulations. Various deviations from the neutral model, including several different forms of natural selection, will inflate the value of Z(nS) relative to its neutral theory expectations. Larger than expected values of Z(nS) are observed in genetic samples from the yellow-ac-scute and Adh regions of Drosophila melanogaster.     

Neocentromere-mediated Chromosome Movement in Maize

Neocentromere activity is a classic example of nonkinetochore chromosome movement. In maize, neocentromeres are induced by a gene or genes on Abnormal chromosome 10 (Ab10) which causes heterochromatic knobs to move poleward at meiotic anaphase. Here we describe experiments that test how neocentromere activity affects the function of linked centromere/kinetochores (kinetochores) and whether neocentromeres and kinetochores are mobilized on the spindle by the same mechanism. Using a newly developed system for observing meiotic chromosome congression and segregation in living maize cells, we show that neocentromeres are active from prometaphase through anaphase. During mid-anaphase, normal chromosomes move on the spindle at an average rate of 0.79 μm/min. The presence of Ab10 does not affect the rate of normal chromosome movement but propels neocentromeres poleward at rates as high as 1.4 μm/min. Kinetochore-mediated chromosome movement is only marginally affected by the activity of a linked neocentromere. Combined in situ hybridization/immunocytochemistry is used to demonstrate that unlike kinetochores, neocentromeres associate laterally with microtubules and that neocentromere movement is correlated with knob size. These data suggest that microtubule depolymerization is not required for neocentromere motility. We argue that neocentromeres are mobilized on microtubules by the activity of minus end–directed motor proteins that interact either directly or indirectly with knob DNA sequences. C urrent models suggest that chromosomes move by a combination of forces generated by microtubule disassembly (Inoue and Salmon, 1995; Waters et al., 1996) and the activity of molecular motors (Vernos and Karsenti, 1996; Yen and Schaar, 1996). Microtubule disassembly generates a constant poleward force; while molecular motors can generate force in either poleward or away-from-pole directions, depending on the characteristics of the motor protein. Both plus and minus end–directed microtubule-based motors are localized to kinetochores (Hyman and Mitchison, 1991). Immunolocalization experiments indicate that mammalian kinetochores contain the minus end– directed motor dynein throughout metaphase and anaphase (Pfarr et al., 1990; Steuer et al., 1990). The kinesin-like proteins CENP-E, which has a transient kinetochore localization in animals, and MCAK, which is localized between the kinetochore plates of mammalian chromosomes, are also thought to generate and/or regulate chromosome movement (Yen et al., 1992; Lombillo et al., 1995; Wordeman and Mitchison, 1995).In addition to the molecular motors on kinetochores, several kinesin-like proteins are localized to chromosome arms (Vernos and Karsenti, 1996). Two subfamilies of arm-based motors have been identified in animals: the NOD subfamily (Afshar et al., 1995; Tokai et al., 1996) and the Xklp1/chromokinesin subfamily (Vernos et al., 1995; Wang and Adler, 1995). Both Nod and Xklp1 are required for positioning chromosomes on the metaphase plate, suggesting that they encode plus end–directed motors (Afshar et al., 1995; Vernos et al., 1995). Other evidence suggests that minus end–directed motors interact with chromosome arms. In the plant Haemanthus, a poleward force acts along chromosome arms during metaphase (Khodjakov et al., 1996), and forces propelling chromosome arms poleward have been detected during anaphase in crane fly spermatocytes (Adames and Forer, 1996). Little is known about how poleward arm motility at metaphase–anaphase affects the fidelity or rate of chromosome segregation.The neocentromeres of maize (Rhoades and Vilkomerson, 1942) provide a particularly striking example of poleward chromosome arm motility. In the presence of Abnormal chromosome 10 (Ab10),¹ heterochromatic DNA domains known as knobs are transformed into neocentromeres and mobilized on the spindle (Rhoades and Vilkomerson, 1942; Peacock et al., 1981; Dawe and Cande, 1996). Knobs are primarily composed of a tandem 180-bp repeat (Peacock et al., 1981) which shows homology to a maize B centromere clone (Alfenito and Birchler, 1993). A characteristic feature of neocentromeres is that they arrive at the spindle poles in advance of centromeres; in extreme cases the neocentromere-bearing chromosome arms stretch towards the poles (Rhoades and Vilkomerson, 1942; Rhoades, 1952). A recently identified mutation (smd1) demonstrates that a trans-acting factor(s) encoded on Ab10 is essential for converting the normally quiescent heterochromatic knobs into active neocentromeres (Dawe and Cande, 1996).Here we use neocentromeres as a model for understanding the mechanisms and importance of nonkinetochore chromosome movement. As a part of our analysis, we developed a four-dimensional system for observing chromosome segregation in living meiocytes. Our experiments were designed to determine (a) how poleward arm motility affects the rate and fidelity of chromosome segregation; and (b) whether the mechanism of neocentromere motility is comparable to the mechanism of kinetochore motility.     

Purification and characterization of two essential cytochromes of the thiosulphate-oxidizing multi-enzyme system from Thiobacillus A2 (Thiobacillus versutus)

Four c-type cytochromes were purified by several procedures including chromatography on DEAE-Sepharose CL-6B, Phenyl-Sepharose CL-4B and Sephadex G-75, G-100 and G-200 and chromatofocusing. Cytochrome c-551 had a pI value of 5.2 and an M_r of 260 000 consisting of six non-covalently bound polypeptides each with an M_r of 43 000, and contained four to five haems. Cytochrome c-552.5 had a pI value of 4.8 and an M_r of 56 000 consisting of two polypeptides with the same M_r 29 000, and contained two haems. Cytochromes c-551 and c-552.5 were reduced by ascorbate to about 70 and 60% of the fully dithionite-reduced values, respectively, and both were essential components in the thiosulphate-oxidizing multi-enzyme system (other components of the system were ‘enzyme A’, ‘enzyme B’ and sulphite: cytochrome c oxidoreductase). These two cytochromes functioned as electron carriers and effectors in the oxidation of thiosulphate. Some evidence suggested that cytochrome c-551 might be a specialized electron transfer component for sulphonate-sulphur oxidation. Both cytochromes could be reduced by thiosulphate in the presence of enzymes A and B. Cytochrome c-550 (basic) and cytochrome c-550 (acidic) were small proteins with M_r 15 000 and 14 000 and pI values of over 8 and 5, respectively. Their physiological role is uncertain.     

The identification of membrane glycocomponents in polyacrylamide gels: A rapid method using I-labeled lectins

A rapid method is described for the identification of lectin binding membrane glycocomponents in polyacrylamide gels. The method requires only small quantities of membrane material and is applicable to a wide variety of lectins. Solubilized membrane components are electrophoretically separated according to molecular weight in a SDS-polyacrylamide gel. The gel is then incubated in a solution containing the ¹²⁵I-lectin. After elution of unbound ¹²⁵I-lectin, the gel is dried down and those bands which have bound the ¹²⁵I-lectin are identified by radioautography. The amount of bound ¹²⁵I-lectin can be quantified by either densitometric scanning of the radioautogram or by liquid scintillation counting of the dried gel.     

Identification of the primate papovavirus HD as the stump-tailed macaque virus.

The recently isolated primate papovavirus HD is shown to be indistinguishable from the stump-tailed macaque virus by immunofluorescent reactivity, by restriction endonuclease analysis, and by nucleic acid hybridization assay.     

Evidence on the conformation of HeLa-cell 5.8S ribosomal ribonucleic acid from the reaction of specific cytidine residues with sodium bisulphite.

The reaction of HeLa-cell 5.8S rRNA with NaHSO3 under conditions in which exposed cytidine residues are deaminated to uridine was studied. It was possible to estimate the reactivities of most of the 46 cytidine residues in the nucleotide sequence by comparing 'fingerprints' of the bisulphite-treated RNA with those of untreated RNA. The findings were consistent with the main features of the secondary-structure model for mammalian 5.85S rRNA proposed by Nazar, Sitz, & Busch [J. Biol. Chem (1975) 250, 8591--8597]. Five out of six regions that are depicted in the model as single-stranded loops contain cytidine residues that are reactive towards bisulphite at 25 degrees C (the other loop contains no cytidine). The cytidine residue nearest to the 3'-terminus is also reactive. Several cytidines residues that are internally located within proposed double-helical regions show little or no reactivity towards bisulphite, but the cytidine residues of several C.G pairs at the ends of helical regions show some reactivity, and one of the proposed loops appears to contain six nucleotides, rather than the minimum of four suggested by the primary structure. Two cytidine residues that are thought to be 'looped out' by small helix imperfections also show some reactivity.     

Effect of sodium chloride on the intracellular solute pools of Listeria monocytogenes.

The concentrations of intracellular solutes in Listeria monocytogenes were examined in cells grown at various concentrations of NaCl. At 5% NaCl, cells contained elevated concentrations of potassium and glycine betaine compared with concentrations in cells grown without NaCl. At 7.5% NaCl, cells contained increased concentrations of K+, glycine betaine, glycine, alanine, and proline. Only glycine betaine, choline, or glycine promoted growth on a solidified defined medium containing 4% NaCl; there was no growth at higher concentrations of NaCl in the defined medium.