AbstractIn quiescent human fibroblasts stimulated to proliferate by fresh medium plus 15 serum, no changes were seen in the incorporation of3H‐tryptophan into the protein of nuclear ribonucleoprotein during the first three hours following re‐feeding. This was in contrast to non‐histone chromosomal proteins where the incorporation increased by 90 within ten minutes. The density of the formaldehyde fixed nuclear ribonucleoprotein in CsCl was 1.43–1.44 g/ml and this also did not change following stimulation. The electrophoretic profile of the proteins of nuclear ribonucleoprotein on SDS gels exhibited a predominant band corresponding to a molecular weight of 44,000 closely trailed by a band at 47,000 and other bands at higher molecular weight. This pattern was not altered by serum stimulation and the same was true for the more complex electrophoretic profile of the chromatin proteins. Following a 10‐minute pulse of3H‐tryptophan at ten minutes after stimulation, there was a selective increase in the labeling of non‐histone chromosomal protein of molecular weight 59,000; no change was seen in the labeling of any protein of nuclear ribo
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