The relation of protein synthesis regulation and RNA metabolism.

Abstract

ABSTRACT The mechanism for regulating protein synthesis in mammalian cells appears to be fundamentally different from that in bacteria. The protein synthesis system of bacteria is largely regulated by the supply of a short-lived messenger RNA and thus is controlled at the level of transcription. In contrast, the messenger RNA of mammalian cells is very long-lived and thus does not appear to vary in amount in response to different demands for protein synthesis. There appears to be a fundamental control system operating at the level of the initiation of translation. The lifetime of messenger RNA in growing HeLa cells is measured directly. There are two components. Forty per cent of the messenger RNA has a half-life of approximately 7 h, while the remaining 60 % has a half-life of from 21 to 24 h. The decay of messenger RNA in actinomycin is somewhat more rapid, but still much slower than the decay of protein synthesis in the presence of the drug. The failure of protein synthesis, caused by actinomycin, appears due to an inhibition of the factor necessary for ribosome association with messenger RNA. Cells can respond to depressed rates of protein synthesis by producing a factor which promotes the association of ribosomes with messenger RNA. This is demonstrated by inhibiting the initiation of translation at elevated temperatures. The cell response, which corrects the lesion, is inhibited by actinomycin but not by cycloheximide. Incubation in cycloheximide alone apparently induces the factor and makes cellular protein synthesis resistant to high temperature. The production of a factor stimulating initiation is demonstrated in vitro. Prior incubation of cells in cycloheximide leads to greatly enhanced rates of initiation in extracts obtained from those cells. Since the cell response is inhibited by actinomycin but induced by cycloheximide, it is concluded that the factor promoting initiation may be RNA.