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-3H 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-3H. 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.

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