Effect of cycloheximide on the synthesis and processing of 5S RNA in HeLa cells

The synthesis and incorporation into ribosomes of 5S RNA were investigated under conditions of protein synthesis inhibition by cycloheximide. Within 1 h after addition of cycloheximide to the culture medium, the synthesis of 5S RNA in HeLa cells, as measured by the pulse labeling of total 5S RNA, was inhibited by 25-30 per cent, but the degree of inhibition did not further increase for several hours. The synthesis of 4S RNA was less markedly inhibited, especially at earlier time periods. Although the appearance of labeled 5S RNA in cytoplasmic ribosomoes after chase was inhibited by approximately 60% or more this presumably reflects the inhibition of 60S ribosomal subunit formation by cycloheximide. Some utilization of preformed 5S RNA for new ribosomes was suggested after resumption of protein synthesis. In confirmation of the results of Kumar and Wu (1), a low dose of actinomycin D inhibited the appearance of 28S RNA in the cytoplasm rather specifically, whereas cycloheximide showed an apparently opposite differential effect.