Polysomal RNA was isolated from subcutaneous myeloma tumors in BALB/c mice after injection of (5-3H)uridine directly into the tumors. Analysis of the polysomal RNA by polyacrylamide gel electrophoresis revealed one major peak of radioactive RNA in the region between 5-S and 18-S RNA with properties consistent with its identification as a messenger RNA. Three pieces of evidence indicated that the species of RNA was probably not myeloma messenger RNA but was histone messenger RNA: 1. 1. The amount of radioactivity in this peak was about the same for two myeloma cell lines that produce widely different amounts of light chains. 2. 2. A peak of radioactive RNA was observed in the same position on the acrylamide gel if polysomal RNA from HeLa cells was isolated and analyzed with methods similar to those used with the myeloma cells. 3. 3. Treatment of tumor-bearing mice with 100 μg/g of cytosine arabinoside inhibited incorporation of [5-3H]uridine into this peak of RNA by more than 75 %.
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