TRANSCRIPTIONALLY ACTIVE CHROMATIN IN LOOPS OF LAMPBRUSH CHROMOSOMES AT PHYSIOLOGICAL SALT CONCENTRATIONS AS REVEALED BY ELECTRON-MICROSCOPY OF SECTIONS

  • 1 January 1981
    • journal article
    • research article
    • Vol. 24  (2) , 298-308
Abstract
The structural organization of the transcribed loops of lampbrush chromosomes present in the vegetative nuclei of the green algae, Acetabularia mediterranea and A. crenulata, and in oocyte nuclei of the newt, Pleurodeles waltlii, was studied by EM of relatively thick (100-200 nm) and ultrathin sections through chromosomes prepared and fixed at physiological salt concentrations. This allows the direct comparison of the same chromosome or chromosome region by EM and light microscopy, that means identification of the transcriptional arrays in specific loops. The loop axis reveals regions smoothly-contoured (non-beaded) and only 4-7 nm thick and are clearly different from supranucleosomal forms of inactive chromatin fibrils as well as from extended filaments of nucleosomal granules examined in parallel. The chromatin of the loop axis of intensely transcribed regions evidently is in a structural form different from that of non-transcribed chromatin. A similarly thin axis was identified in loops of chromosomes of nuclei fixed in situ. The lateral ribonucleoprotein (RNP) fibrils associated with transcribed loop regions appear as serial arrays of granules, often of regular size, which are smaller in the chromosomes of Acetabularia (mean diameter 18 nm) than in those of amphibia (mean diameter 28 nm). Discontinuous arrays of lateral RNP fibrils and fibril arrays with different polarity are found in some loops. Certain loops and loop regions are characterized by specific patterns of aggregation of such lateral RNP fibrils which appear to correspond to the granules visible in these loops in the light microscope. Arrays of transcriptional complexes in chromosome loops can be visualized in thin sections of material prepared at physiological ionic strength with similar resolution and clarity as in spread preparations of chromosomal material dispersed in extremely low salt buffers. The organization of loop structures in vivo show that, at physiological ionic strength, the chromatin of the loop axis is organized in a form different from that characteristic of non-transcribed chromatin.

This publication has 1 reference indexed in Scilit: