Organizational patterns within chromosomes

Improved Production of Streptomyces sp. FA1 Xylanase in a Dual-Plasmid Pichia pastoris System

Methanol is considered as a potential hazard in the methanol-induced yeast expression of food-related enzymes. To increase the production efficiency of recombinant proteins in Pichia pastoris without methanol induction, a novel dual-plasmid system was constructed, for the first time, by a combining the strategies of genomic integration and episomal expression. To obtain a high copy number of the target gene, the autonomously replicating sequence derived from Kluyveromyces lactis (PARS) was used to construct episomal vectors carrying the constitutive promoters P_GAP and P_GCW14. In addition, an integrative vector carrying the P_GCW14 promoter was constructed by replacing the P_GAP promoter sequence with a partial P_GCW14 promoter. Next, using xylanase XynA from Streptomyces sp. FA1 as the model enzyme, recombination strains were transformed with different combinations of integrating and episomal vectors that were constructed to investigate the changes in the protein yield. Results in shake flasks indicated that the highest enzyme yield was achieved when integrated P_GAP and episomal P_GCW14 were simultaneously transformed into the host strain. Meanwhile, the copy number of xynA increased from 1.14 ± 0.46 to 3.06 ± 0.35. The yield of XynA was successfully increased to 3925 U·mL^-1 after 102 h of fermentation in a 3.6 L fermenter, which was 16.7-fold and 2.86-fold of the yields that were previously reported for the constitutive expression and methanol-induced expression of the identical protein, respectively. Furthermore, the high-cell-density fermentation period was shortened from 132 h to 102 h compared to that of methanol-induced system. Since the risk of methanol toxicity is removed, this novel expression system would be suitable for the production of proteins related to the food and pharmaceutical industries.

A correlated light and electron microscope study of the nucleolar material during mitosis in Vicia faba

Root meristematic cells of Vicia faba were examined, with both light and electron microscopes, in order to study the behaviour of the nucleolar material during the mitotic process. Under light microscopy, the preprophase nucleolus is seen to consist of a densely stained material in which are embedded several unstained vacuole-like structures of varying size. The electron microscope reveals that the dense nucleolar material is formed of two structurally distinct components, each segregated into irregularly shaped zones blending with one another. One of these components is represented by 150 A granules which, in places, are arranged into thread-like structures approximately 0.1 µ in diameter; the other component apparently consists of fibrils 60 to 100 A in diameter. The large and medium sized intranucleolar vacuoles contain loosely scattered granules and fibrils similar to those just described. The granular and fibrillar components of the denser portion of the nucleolus persist as such during prophase and disperse throughout the nuclear cavity at the time of nucleolar disintegration. After nuclear membrane breakdown, these granules and fibrils, as well as those of the nucleoplasm, mix freely with similar elements already present within the forming spindle. No evidence has been obtained that, during or after nucleolar disintegration, the structural components of the nucleolus become associated as such with the chromosomes to form an external or internal matrix. Our observations suggest the existence, of a matrix substance within late prophase, metaphase, and anaphase chromosomes, the fine structure of which bears strong resemblance to that of their constituent coiled chromonemata. Data are presented, moreover, that indicate that part of this matrix substance, presumably formed at some time during prophase, is released from the chromosomes during their anaphasic movement. A number of observations indicate that the main bulk of the next nucleolus is derived from a prenucleolar fibrillogranular material, arranged into thread-like structures some 0.1 µ in diameter, which collect in the interchromosomal spaces during early and midtelophase. Finally, our data would seem to favour the view that most of this prenucleolar material results from a resumption of the synthetic activity of the early and midtelophase chromosomes rather than from a mere shedding of a preexisting matrix substance.

Electron microscope studies on salivary gland cells. I. The nucleus of Bradysia mycorum Frey (Sciaridae), with special reference to the nucleolus

Salivary glands were fixed in cold 1 per cent osmium tetroxide in veronal-acetate buffer containing sucrose and embedded in methacrylate mixture or Araldite. The salivary gland nuclei of sciarids show a continuous production of nucleoli, which remain multiple and not consolidated into a single structure. The earliest recognizable nucleoli, which we call "elementary nucleoli," are aggregations of a few paired 40 A fibrils and a few 150 A particles, at many points within chromosome bands. Further development consists of the detachment of the elementary nucleoli from their points of origin and their subsequent mutual coalescence. As a result, dense patches of nucleolar material are formed which become large nucleoli at the surface of chromosomes, either attached to the band or free. The fully formed nucleoli have a characteristic dual structure with a narrow dense periphery and a broader less dense internum. Fibrils and particles are present in both regions, and the difference in density reflects differences in the packing of the two structural elements. The duality in structure is lost in later stages. The nucleolar fibrils appear to be similar to the chromosomal fibrils. The 150 A particles in nucleoli, chromosomes, and nuclear sap seem identical. The significance of these observations is discussed for nucleologenesis in general.

A study of newt mitotic chromosomes by negative staining

A method is described for bursting single, selected mitotic cells on a fluid surface. Cells from cultures of newt heart tissue were burst on dilute solutions containing potassium and sodium with and without added calcium and also on dilute calcium chloride solution. The material was negatively stained with uranyl acetate or sometimes with ammonium molybdate or sodium phosphotungstate. The bodies of chromatids spread on NaCl/KCl solutions showed many parallel fibers about 150 A in diameter. Loops with a complex nodular structure were observed projecting from the sides and ends of chromatids. In calcium-containing solutions there was evidence of fiber coagulation; the chromatid body was more compact and laterally projecting fibers tended to be pulled out straight. Especially in the absence of calcium the chromosomal fibers had a nodular form and appeared to be composed of irregularly folded fibrillar elements. The question as to whether chromosomal fibers, which range in diameter from about 50 to 300 A, consist of single, folded threads or of two or more adjacent subunits is discussed.

The strandedness of meiotic chromosomes from Oncopeltus

Meiotic chromosomes were isolated from male Oncopeltus fasciatus by dissecting the testes under insect Ringer's solution and spreading the living cells on the Langmuir trough. After being dried by the critical point method, preparations were examined under the electron microscope. Chromosomes at all stages of prophase prove to be multistranded. A significant increase in the number of parallel 250 A fibers in the chromosomes occurs between zygotene and diakinesis. Parallel folding, rather than true multistrandedness, is interpreted as the mechanism responsible for this observed increase in multistrandedness. It has not been possible to determine whether the multistrandedness observed at leptotene represents true multistrandedness or is the result of parallel folding. Apparent multistrandedness is lost at metaphase when the 250 A fibers of the chromosomes become coiled more tightly. In preparations isolated by these methods, no structures other than the 250 A chromosome fibers are visible in the chromomeres, which appear as regionally coiled or folded areas of the fibers along the arm of the chromosome.

Incorporation of tritium-labeled thymidine and lysine into chromosomes of cultured human leukocytes

The incorporation of thymidine-H(3) and lysine-H(3) into human leukocyte chromosomes was studied in order to determine the temporal relationships between the syntheses of chromosomal deoxyribonucleic acid and chromosomal protein. The labeled compounds were incorporated into nuclei of interphase cells. Label from both precursors became apparent over the chromosomes of dividing cells. Incorporation of thymidine-H(3) occurred during a restricted period of midinterphase (S) which was preceded by a nonsynthetic period (G(1)) and followed by a nonsynthetic period (G(2)). Incorporation of lysine-H(3) into chromosomal protein occurred throughout interphase. Grain counts made over chromosomes of dividing cells revealed that the rate of incorporation of lysine-H(3) into chromosomal protein differed during various periods of interphase. The rate of incorporation was diminished during G(1). During early S period the rate of incorporation increased, reaching a peak in late S. The high rate continued into G(2). Thymidine-H(3) incorporated into DNA was distributed to mitotic chromosomes of daughter cells in a manner which has been referred to as a "semi-conservative segregation." No such semi-conservative mechanism was found to affect the distribution of lysine-H(3) to the mitotic chromosomes of daughter cells. Therefore, it is concluded that synthesis of chromosomal protein and its distribution to chromosomes of daughter cells are not directly influenced by synthesis and distribution of the chromosomal DNA with which the protein is associated.

Strandedness of Vicia faba chromosomes as revealed by enzyme digestion studies

Chromosomes and nuclei isolated from neutral formalin-fixed Vicia faba lateral roots were treated with trypsin, pepsin, RNase, or DNase. Only trypsin affected the morphology of the chromosomes and nuclei. The appearance of the chromosomes after trypsin digestion indicated that each chromatid contained four strands that could be seen with an ordinary light microscope. The experiments are interpreted as indicating that mitotic chromosomes of Vicia faba are multistranded and that the linear continuity of the chromosome is dependent on protein.