Wasting of 18 S ribosomal RNA by human myeloma cells cultured in adenosine

When human myeloma cells are pulsed for one hour with 3H-uridine and chased for six hours in fresh medium containing unlabeled uridine, the processing of 45 S rRNA precursor into the stable 28 S and 18 S rRNA components can be followed. However, when the cells are chased in exogenous adenosine instead of uridine, the accumulation of 18 S rRNA is selectively inhibited. Cells pulsed with 3H-adenosine and chased in the absence of exogenous nucleosides exhibit normal rRNA precursor processing, while cells pulsed simultaneously with 3H-uridine and 3H-adenosine and chased with uridine and adenosine are deficient in labeled 18 S rRNA. Consequently, the inhibition of 18 S rRNA accumulation by adenosine is not an artifact of labeling nor is it relieved by an equal molar concentration of uridine. The wasting of 18 S rRNA in human myeloma cells is similar to that reported to occur in normal lymphocytes during the quiescent state.