Fibroblasts transformed by an oncogenic virus show decreased uptake of puromycin aminonucleoside

Effect of aminonucleoside on transcription, methylation, and maturation of ribosomal RNA in SV40-transformed human lung fibroblasts

The effects of puromycin aminonucleoside (AMS) have been studied in the nucleoli of SV40-transformed human lung fibroblasts with attention to the relationship between transcription, methylation, and maturation of ribosomal RNA (rRNA). Inhibition of the transcription of pre-rRNA became evident between 1 and 2 hr of exposure to AMS, and the degree of inhibition remained approximately constant throughout the remainder of the 18-hr period of study. Methylation of the ribose units of pre-rRNA was inhibited by AMS approximately as much as RNA transcription, but later, suggesting that the inhibition of methylation is a consequence of the lowered rate of RNA transcription. It was also noted that ribose methylation of nucleolar RNA occurs in the absence of concurrent RNA transcription, indicating that rRNA may be methylated post-transcriptionally. Exposure of the fibroblasts to fresh serum had the capacity to increase the rate of nucleolar RNA transcription if the fibroblasts were treated with AMS for periods of less than 2 hr but was ineffective thereafter, confirming that the inhibitory effect of AMS on pre-rRNA transcription is established in approximately 2 hr. On the other hand, processing of the primary transcript (45 S RNA) to the mature rRNA species (28 and 18 S) was inhibited more gradually, with a complete inhibition of rRNA maturation being noted after 18 hr of AMS treatment. These data are consistent with the view that the primary effect of AMS is on the rate of rRNA transcription, with a later effect on its maturation, while RNA methylation is reduced only because of the diminished availability of the RNA substrate.