Degradation of polyribosomes in the liver of rats treated with 5-azacytidine (25 mg/kg body weight) was a reversible process. Accumulation of the fraction containing mono- and disomes ceased 18 to 20 hr after the drug was administered. The level of charged leucyl transfer RNA attached to liver polyribosomes following 5-azacytidine treatment decreased for about 20 hr after administration of the analog, regardless of polyribosome size. In accordance with the enhanced level of higher polyribosomal aggregates observed 24 hr after drug administration, the binding of leucyl transfer RNA both to heavier polyribosomes (n ⩾ 3) and to the fraction of monosomes and disomes was also enhanced. The labeling of nonribosomal RNA attached to individual oligosomes in liver cytoplasm was not inhibited during the course of their degradation induced by 5-azacytidine. In cultured Novikoff hepatoma cells, 5-azacytidine completely blocked the maturation of ribosomal RNA. This effect is due to the inhibition of the processing of 45 S precursor RNA, the formation of 28 S and 18 S RNA being the most sensitive to the presence of the analog. The degradation of polyribosomes occurring in the liver of 5-azacytidine-treated rats is discussed in relation to the impaired maturation of ribosomal RNA.
Portions of the work were supported by Grant CA-07175 from the National Cancer Institute and Grant E-588 from the American Cancer Society.