RNA TRANSPORT FROM NUCLEUS TO CYTOPLASM IN CHIRONOMUS SALIVARY GLANDS

Abstract

The fine structure and cytochemistry of the extremely large RNA puffs, or Balbiani rings, in salivary gland nuclei of midge, Chironomus thummi, larvae have been investigated. The Balbiani rings are composed of a diffuse mass of electron-opaque 400 to 500 A granules, short threads about 180 to 220 A in diameter and associated fine chromatin fibrils. These components appear to be organized into brushlike elements which form the ring. Electron microscope cytochemistry has shown that the granules and short threads contain RNA. After ribonuclease digestion, only 50 to 100 A chromatin fibrils were apparent in the Balbiani ring, and the granules were no longer demonstrable. Deoxyribonuclease digestion had no apparent effect on these structures. Observations indicate that the granules are formed from the short threads and released into the nucleoplasm in which they are evenly distributed. At the nuclear envelope, many granules have been observed partially or completely within the nuclear pores. These granules become elongated and are shown to penetrate the center of the pore in a rodlike form, about 200 A in diameter. The Balbiani ring granules are not normally visible within the cytoplasm adjacent to the nuclear envelope, but have been rarely found in this region. It is suggested that the granules represent the product of the Balbiani ring, possibly a messenger RNA bound to protein, and that they regularly pass into the cytoplasm through a narrow central channel in the pores of the nuclear envelope.

The Ultrastructure of the Nuclear Envelope of Amphibian Oocytes: A Reinvestigation

“…These helices can be frequently observed during their passage through the innermost part of the nuclear pore lumen and thus are structures comparable to the central granules . An RNP-nature of the central granule is furthermore indicated by the demonstrations of the material derived from Balbiani-rings in Chironomus salivary glands (3,36), from lampbrush loops in amphibian eggs (37) and from the nucleolar periphery in diverse oocytes (2,10,21,22,31) A further step in examining the hypothesis that the central granule is RNA-containing material on its transit from nucleus to cytoplasm, now, would be to make use of the inhibition gap selective for the synthesis of rRNA and that of tRNA and mRNA . Thus it might be possible to clarify whether the central granule is identical to ribosomal or preribosomal RNP-material which is by far predominantly synthesized in the lampbrush stage of amphibian oogenesis (5,6,30) .…”

“…(b) In synchronized cultures of the ciliate Tetrahymena pyriformis a correlation exists between physiological states of high RNA-synthesis and the frequency of granules in the macronuclear pores (42) . (c) Cross-sections through the nuclear envelopes of cells highly active in RNA-synthesis often show dense dumbbell-shaped clumps of material, presumably of RNP nature, lying on either side of the pore complex and connected by an about 100-150 A broad rod (2,10,21,31,36) . Such configurations are widely interpreted as material in transit through the "central channel" of the pore and obviously can correspond to the appearance of the central granules in the tangential sections and in the negatively stained preparations of isolated nuclear envelopes.…”

Messenger RNA processing and nuclear structure: isolation of nuclear ribonucleoprotein particles containing beta-globin messenger RNA precursors.

“…This fact was first suggested by light microscope studies of nascent RNA on the lateral loops of amphibian oocyte lampbrush chromosomes (9,16), and has been more dramatically demonstrated by the electron microscope identification of hnRNA:RNP particles in situ (29,32,45) and on unfolded chromatin fibers spread from nuclei or cells by detergent lysis (8,26,27,31). Initial attempts to extract these hnRNP particles from nuclei resulted in the isolation of degraded, 40S RNP complexes (43), because of the action of endogenous nucleases.…”

On the Attachment of the Nuclear Pore Complex

“…The roles which have been postulated for the pore and the complex fall primarily into two mutually compatible classes: (a) involvement in nucleo-cytoplasmic communication (9)(10)(11)(12), and (b) organization of the interphase chromatin (13 16). Little direct evidence has been obtained to support either role, and there is only fragmentary information concerning the chemical composition of the complex (17,18).…”