Cycloheximide is not a specific inhibitor of protein synthesis in vivo.

Cycloheximide is frequently presumed to inhibit specifically the cytoplasmic protein synthesis of eukaryotes. Although previous investigators have shown that it had other effects on the cells of a variety of organisms, these results were frequently presumed to be secondary effects of the inhibition of protein synthesis. This paper shows that a wide range of deleterious effects are produced by cycloheximide on a single organism, Chlamydomonas reinhardi Dangeard. If, protein synthesis is inhibited by nonpermissive conditions in temperature-sensitive mutants or with other treatments these "secondary" effects are not produced. Instead, cycloheximide appears to have two or three independent inhibitory effects on the cell. Moreover, in contrast to a number of previous investigations, these results show that protein synthesis is not required for RNA synthesis. Instead the rate of RNA synthesis is actually increased by interference with protein synthesis.