EFFECT OF PROTEOLYTIC ENZYMES ON SEDIMENTATION PROPERTIES OF RIBONUCLEOPROTEIN PARTICLES FROM HEART MUSCLE

Drosophila polyribosomes. The characterization of two populations by cell fractionation and isotopic labeling with nucleic acid and protein precursors

Two populations of polyribosomes have been isolated from third instar larvae of D. melanogaster. One population appeared to be soluble while the second seemed membrane-bound. Short-term labeling of the two RNP fractions with radioactive nucleic acid and protein precursors was achieved by using a feeding stimulant. RNA was extracted from both polyribosomal fractions following 25, 40, and 60 min of in vivo uridine-(3)H incorporation. Soluble polyribosomes exhibited more rapid uptake of uridine into ribosomal and heterogeneous RNA fractions than did membrane-bound polyribosomes at comparable time periods. In vivo amino acid incorporation into the two polyribosomal populations was examined after 10, 20, 40, 60, and 80 min of incubation in leucine-(3)H. In this case, the membrane-bound polyribosomes reached a higher specific activity than did the soluble ones. These functional differences confirmed the observation, based on cellular fractionation studies, that the two classes of polyribosomes represented functionally distinct populations. These data have been compared with those from studies on other metazoan systems. In addition, dithiothreitol has been demonstrated to be a powerful ribonuclease inhibitor.

The effect of ribonuclease on polysomes and ribosomes of bacteria and animal cells

The mildest treatment with ribonuclease that causes any disaggregation of the polysomes of Escherichia coli or HeLa cells simultaneously attacks the RNA of the constituent ribosomes. It is concluded that the susceptibility to ribonuclease of polysomes does not suggest that they are held together by a strand of messenger RNA. The RNA of the larger sub-unit of bacterial ribosomes has particularly sensitive regions resulting in a non-random degradation. The RNA of the smaller sub-unit of E. coli ribosomes is relatively resistant to ribonuclease attack. The same may be true of the respective sub-units of the intact HeLa-cell ribosome, but both sub-units become very sensitive to ribonuclease on dissociation from each other.

Arrested protein synthesis in polysomes of cultured chick embryo cells

Cells deprived of serum synthesize proteins at a reduced rate; when serum is restored the rate returns to normal. The polysomes do not dissociate, but show reduced incorporation of amino acid in vitro, and are less responsive to polyuridylic acid than are those from normal cells.