Isolation and characterization of collagen-synthesizing polysomes from chick embryos.

Collagen-synthesizing polysomes were isolated by low-speed centrifugation of the post-mitochondrial supernatant of chick homogenates. Electron microscopy of the fraction thus isolated shows it to be exclusively composed of ribosomes. Amino acid incorporation in vitro showed that these particles were efficient in the incorporation of proline, but not tryptophan, as opposed to ribosomes obtained from the supernatant of the low-speed centrifugation. The incorporation process was highly dependent on GTP, and exibited an optimal Mg2+concentration of 5.6mM. The reaction was inhibited by RNase, elongation inhibitors as anysomycin, sparsomycin, fusidic acid and GDPCP. It was also moderately inhibited by initiation inhibitors such as aurintricarboxilic acid and pyrocatechol violet. The product of the incorporation was characterized as collagen by its sensitivity towards purified collagenase, lack of tryptophan, chromatography in CM-cellulose and molecular sieve chromatography in Sephadex G-200.     

Synthesis of exported proteins by membrane-bound polysomes from Escherichia coli.

A membrane-bound fraction of polysomes of Escherichia coli has been isolated after lysis of cells without the use of lysozyme. Protein-synthesis studies in vitro show that membrane-bound and free polysomes are different in the following respects. 1. Membrane-bound polysomes synthesize proteins which are exported from the cell. The products include proteins of the outer membrane and a secreted periplasmic protein, the maltose-binding protein. 2. The major product synthesized by free polysomes is elongation factor Tu, a soluble cytoplasmic protein. 3. The activity of membrane-bound polysomes in vitro is more resistant to puromycin than is the activity of free polysomes. In addition, the mRNA associated with membrane-bound polysomes is more stable than the bulk of cellular mRNA as revealed by studies with rifampicin.     

A rapid method for isolation of peptidyl tRNA from liver polysomes.

A rapid method for the isolation of peptidyl tRNA from mouse liver polysomes is described. Peptidyl tRNA was isolated from the protein pad which forms at the interface of the aqueous and organic layers during extraction of RNA from ribosome samples with chloroform-isoamyl alcohol. Peptidyl tRNA was readily detected by electrophoresis in acrylamide gels and staining with Coomassie blue or methylene blue.     

Free and membrane-bound polysomes from rat liver. 1. Improvements of subcellular fractionation

Substantial improvements of cellular fractionation in ionic conditions allowing preservation of polysome structure and polysome-membrane interactions are reported. They consist primarily in minimizing the lysosomal content of fractions containing endoplasmic reticulum by isolating a lysosome-rich fraction with little loss (6%) of RNA. Endoplasmic reticulum membranes were recovered in high yield, mainly in association with mitochondria, the remainder being found in the post-lysosomal supernatant. The latter also contained practically all the free polysomes, as judged by metrizamide gradient analyses. The distributions of various constituents (RNA, DNA, protein and marker enzymes) among cell fractions is presented.     

Isolation of serum albumin-synthesizing polysomes from rat liver

The procedures for the purification of rat liver polysomes synthesizing serum albumin was developed, employing the quantitative precipitin method with rat serum albumin as a carrier and its antibody, and ribonuclease inhibitor from rat liver. The addition of ribonuclease inhibitor to polysomes during the incubation with antibody was found to prevent their degradation. Under these conditions, about 12 % of the membrane-bound polysomes of rat liver was found in the specific precipitate of serum albumin and its antibody, while a negligible amount of free polysomes was precipitated. It is concluded that polysomes synthesizing serum albumin are isolated by this method.     

Isolation of myosin-synthesizing polysomes from cultures of embryonic chicken myoblasts before fusion.

Mononucleated myoblasts and multinucleated myotubes were obtained by culturing embryonic chicken skeletal muscle cells. Comparison of total polysomes isolated from these mononucleated and multinucleated cell cultures by density gradient centrifugation and electron microscopy revealed that mononucleated myoblasts contain polysomes similar to those contained by multinucleated myotubes and large enough to synthesize the 200,000-dalton subunit of myosin. When placed in an in vitro protein-synthesizing assay containing [³H]leucine, total polysomes from both mononucleated and multinucleated myogenic cultures were active in synthesizing polypeptides indistinguishable from myosin heavy chains as detected by measurement of radioactivity in slices through the myosin band on sodium dodecyl sulfate (SDS)-polyacrylamide gels. Fractionation of total polysomes on sucrose density gradients showed that myosin-synthesizing polysomes from mononucleated myoblasts may be slightly smaller than myosin-synthesizing polysomes from myotubes. Multinucleated myotubes contain approximately two times more myosin-synthesizing polysomes per unit of DNA than mononucleated myoblasts, and the proportion of total polysomes constituted by myosin polysomes is only 1.2 times higher in multinucleated myotubes than it is in mononucleated myoblasts. The results of this study suggest that mononucleated myoblasts contain significant amounts of myosin messenger RNA before the burst of myosin synthesis that accompanies muscle differentiation and that a portion of this messenger RNA is associated with ribosomes to form polysomes that will actively translate myosin heavy chains in an in vitro protein-synthesizing assay.     

Messenger ribonucleoprotein complexes isolated by oligodeoxythymidylate-cellulose chromatography from Neurospora crassa polysomes.

Messenger ribonucleoprotein (mRNP) complexes were isolated from ethylenediaminetetraacetic acid-dissociated polysomes of Neurospora crassa. Approximately 15% of the [3H]uridine incorporated into polysomal ribonucleic acid (RNA) during a 15-min pulse was eluted from oligodeoxythymidylate-cellulose as an mRNP complex. The isolated mRNP complexes exhibited sedimentation coefficients ranging from 15S to greater than 60S. RNA isolated from these mRNP complexes sedimented in sucrose gradients between 4S and 40S, with broad peaks at 15S and 24S. The buoyant density of mRNP complexes eluted with 25% formamide was 1.42 to 1.44 g/cm3, whereas for mRNP complexes eluted with 50% formamide it was 1.48 to 1.50 g/cm3. Six polypeptides, with molecular weights of 14,000, 19,000, 24,000, 31,000, 44,000, and 66,000, were associated with mRNP complexes eluted with 25% formamide. The mRNP complexes eluted with 50% formamide had one associated polypeptide, of molecular weight 27,000.