. Biological structure and function; proceedings. Biochemistry; Cytology. NUCLEAR PROTEIN SYNTHESIS 269 ditions (upper curve); ribonucleoprotein particles isolated in, or exposed to hypertonic sucrose solutions are completely inert (lower curve). This finding is in agreement with earlier experiments on intact nuclei showing that amino acid uptake proceeds best in isotonic sucrose solutions and is impaired when nuclei are exposed to either hypertonic or hypotonic conditions [44]. Leucine-l-C , , Isotonic nuclear nbosomes. Ribosomes after brief hypertonic exposure 20 40 60 Incubation time (min)
. Biological structure and function; proceedings. Biochemistry; Cytology. NUCLEAR PROTEIN SYNTHESIS 269 ditions (upper curve); ribonucleoprotein particles isolated in, or exposed to hypertonic sucrose solutions are completely inert (lower curve). This finding is in agreement with earlier experiments on intact nuclei showing that amino acid uptake proceeds best in isotonic sucrose solutions and is impaired when nuclei are exposed to either hypertonic or hypotonic conditions [44]. Leucine-l-C , , Isotonic nuclear nbosomes. Ribosomes after brief hypertonic exposure 20 40 60 Incubation time (min) Fig. 3. The time course of incorporation of [i-^*C]-leucine into isolated ribonucleoprotein particles prepared from thymus nuclei. The incubation medium contained (in addition to the particles), nuclear pH 5 enzymes. ATP + an ATP- regenerating system, and GTP. The upper curve shows the specific activity of the proteins of the ribosomes under isotonic conditions. A brief exposure to hypertonic sucrose solutions destroys the capacity for amino acid uptake under these conditions (lower curve). The incorporation of amino acids by isolated nuclear ribosomes is inhibited if ribonuclease is added to the incubation medium. This eiTect is of interest because it contrasts with findings on intact nuclei. It has been shown that ribonuclease does not affect amino acid uptake into the ribo- somes of the intact isolated nucleus [37]. The results indicate that the RNA of the ribosome fraction is immune to attack by RNAase as long as it remains inside the nucleus. This is one of the unique properties of the protein synthetic pathway in the nucleus. Some characteristically nuclear aspects of amino acid incorporation On the whole, the study of sub-nuclear fractions has shown that the protein synthetic mechanism in the cell nucleus is very much like that of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and a
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