The effects of methionine deprivation on ribosome synthesis in HeLa cells.

MIany aspects of ribosomal RNA synthesis and ribosome formation in cultured animal cells have been clarified over the past several years. Ribosomal precursor RNA synthesis in the cell nucleolus' 2 is rapidly followed by methylation of the RNA.', 4 Within 20-40 minutes of synthesis this new RNA can be detected in nuclear subribosomal particles which are precursors tocytoplasmicsubribosomal particles.' It thus becomes possible to study in detail the effects of various metabolic restrictions on ribosome formation. The present experiments are concerned with the effects of methionine starvation. In HeLa cells, methionine not only is an essential amino acid for the formation of protein but also furnishes methyl groups in the methylation of nucleic acid bases and of the 2'-OH group of ribose, the major site of methylation in HeLa ribosomal RNA.'7 As will be shown, in the absence of exogenous methionine, complete ribosomes are not formed. Evidence is presented that the degree of methylation of ribosomal precursor RNA is severely limited, and that this may be the factor limiting ribosomal maturation. In spite of this restriction of ribosome completion, the transcription of ribosomal precursor RNA continues. In contrast, cells deprived of valine, another growth-essential amino acid without any special function in the formation of ribosomal RNA, continue to initiate ribosomal RNA synthesis and to complete ribosomes for many hours. This is presumably due to the provision of valine via protein turnover.8