Cyclic variations in protein, DNA and ribosomal RNA synthesis in 6C 3HED lymphoma cell cultures due to essential amino acid depletion

Abstract

Suspension cultures of 6C 3HED lymphoma cells growing at concentrations between 3·10 6 and 5·10 6 per ml develop gross changes in nucleic acid and protein synthesis within 24 h of dilution of the culture with an equal volume of fresh medium. The effect was shown to be due to depletion of one or more of the essential amino acids by the cells as they double in number. Unlike bacteria in “step-down” culture, DNA synthesis is most affected; its rate of synthesis in each cell in S phase declines and there is some retardation of initiation of S. Of only slightly less magnitude is the selective decline in the transcription of rRNA. The maturation of precursor rRNA also is retarded. A much smaller fall in the rate of synthesis of DNA-like RNA, tRNA and 5-S RNA may be due to alterations in the handling by the cells of the exogenous nucleosides used for tracer analysis (pool effects). Following replenishment of the essential amino acids in the medium all these effects are rapidly reversed. There is a rapid increase in protein synthesis followed by an increase in the rate of DNA synthesis, which is greater in magnitude than the concurrent increase in rRNA transcription. There is an increase in the rate of initiation of DNA synthesis. Also there is an acceleration in the processing of rRNA precursors to the mature rRNA components. Despite little evidence of change in the rate of synthesis of DNA-like RNA as a whole, some subtle alterations within this fraction were revealed by methylated bovine serum albumin kieselguhr chromatography. The correlation between the changes in rate of transcription and the maturation of rRNA which accompany changes in the rate of protein synthesis induced by amino acid deficiency or cycloheximide in these cells, and in other mammalian cells suggests that the process of maturation of rRNA may provide a mechanism to modulate the rate of supply of ribosomes to the cell which is dependent on the rate of protein synthesis.