Electron Microscope Studies of Ribosomal Clusters Synthesizing Hemoglobin

107

This communication presents a morphological study of the changes in ribosome content and organization which occur during the maturation of erythroid cells of the phenylhydrazinetreated rabbit. Electron micrographs of thin sectioned nucleated and non-nucleated erythroid cells have been subjected to a quantitative analysis of the distribution of ribosomes as polyribosomes of various sizes and as single ribosomes. The ribosomes of nucleated erythroid cells of marrow are virtually all arranged in the polyribosome configuration consisting of clusters of 2 to 6 individual ribosomes. These cells are the most active in the erythroid series in protein biosynthesis. During maturation to the non-nucleated reticulocyte stage, found in the circulating blood, there is a decrease in protein synthesizing capacity, a fall in total ribosome content, and, more significantly, a decrease in the number and size of polyribosomes. Maturation to the ribosome-free erythrocyte, either under in vitro or in vivo conditions, entails a further decrease in protein synthesis which correlates with a progressive disaggregation of the biosynthetically active polyribosomes into smaller clusters and inactive single ribosomes. Possible models which may account for the stability of the polyribosome and for the mechanism of polyribosome dissociation are discussed.

Section: Structure Of the Polyribosomementioning

confidence: 66%

Section: Introductionmentioning

confidence: 99%

Alterations in Polyribosomes During Erythroid Cell Maturation

Rifkind

Danon

Marks

1964

107

“…7, 8, 10, and 15) may represent polysomes, or groups of ribosomes engaged in protein synthesis (Warner et al, 1962). If so, the existence of plastid "messenger" R N A may be postulated.…”

Section: Discussionmentioning

confidence: 99%

The Fine Structure of Chloroplast Stroma Following Aldehyde Osmium-Tetroxide Fixation

Gunning

1965

126

Markedly improved fixation of leaf tissues is obtained by means of a glutaraldehyde (or acrolein)-osmium tetroxide procedure, as compared with the results of potassium permanganate or osmium tetroxide fixation methods. The procedure has proved useful in all species so far examined. Chloroplasts are particularly well preserved. In this paper details of components of the ground-substance of Avena sativa plastids are presented. They include the following :--(i) The "stromacentre" is an area of aggregated fibrils, each 85 A in diameter, and of uncertain length. Individual fibrils may be composed of subunits. The whole aggregate is usually up to 1 # in diameter, and is visible in thin sections in the light microcope. It is present at all stages of plastid development, and, under conditions of rapid synthesis in the plastid, it may be up to 2 ~ in diameter. Evidence that it is proteinaceous is presented. Osmiophilic globules are often associated with it. (ii) Areas which resemble bacterial and blue-green algal nucleoplasms, containing fibrils approximately 30 A wide. These regions are smaller than the stromacentre and, like that structure, they occur in all stages of plastid development. Unlike it, there are several such areas per chloroplast. (iii) Particles which have some of the morphological and staining characteristics of ribosomes. Present at all stages of development, they are approximately two-thirds the size of the cytoplasmic ribosomes. They can occur in groups, thus resembling polyribosomes. (iv) The remaining material is granular, and may include dissociated portions of stromacentre material. The validity of the observations and their significance is discussed.

“…Since the original studies of polyribosomes (20,10,22), evidence has accumulated that most protein synthesis takes place on a structure, referred to as polyribosomes, which is composed of messenger RNA and a number of ribosomes. We examined mitotic and interphase HeLa cells for the presence of polyribosomal complexes.…”

Section: Hela Cellsmentioning

confidence: 99%

Ribonucleic Acid and Protein Synthesis in Mitotic Hela Cells

Johnson

Holland

1965

109

HcLa cells arrested in mitosis were obtained in large numbers, with only very slight interphase cell contamination, by employing the agitation method of Terasima and Tolmach, and Robbins and Marcus. Protein synthesis and RNA synthesis were almost completely supprcsscd in mitotic cells. Active polyribosomes were nearly absent in mitotic cells as compared with intcrphase cells treated in the same way. Cell-free protein synthesis and RNA polymcrase activity were also greatly depressed in extracts of metaphasc cells. The dcoxyribonuclcoprotein (DNP) of condenscd chromosomes from mitotic cells was less cfficicnt as a template for Escherichia coli RNA polymcrase than was DNP from interphase cells, although isolated DNA from both sources was equally active as a primer. Despite very poor endogenous amino acid incorporation by extracts of mctaphase cells, polyuridylate stimulated phenylalanine incorporation by a larger factor in mitotic cell extracts than it did in interphase cell cxtracts. These rcsults suggest that RNA synthesis is suppressed in mitotic cells because the condensed chromosomes cannot act as a template, and that protein synthesis is deprcsscd at least in part because messenger RNA becomes unavailable to ribosomes. This conclusion was supported by the demonstration that cells arrested in mctaphase supported multiplication of normal yields of poliovirus, thereby showing that the mitotic cell is capable of considerable synthesis of RNA and protein.