Entry of radioactivity from methyl‐labeled methionine into mitochondrial and cytoplasmic RNA of cultured hamster cells

Following the work of Mandel and Borek [l] most analyses of post-transcriptional methylation of nucleic acid have employed methyl-labeled methionine as a specific precursor, via S-adenosylmethionine (SAM), of RNA or DNA methyl groups. For example, we as well as others have relied on this approach in studies comparing cytoplasmic and mitochondrial nucleic acids in cultured mammalian cells [2-61. The validity of such studies depends on the assumption that, if separate SAM pools exist in the different sub-cellular compartments, they are labeled with comparable efficiency by exogenous methionine. This assumption has recently been questioned in a report on mitochondrial DNA [2] , and there is evidence from a chick embryo system that compartmentalization of SAM can affect apparent methylation levels of cytoplasmic ribosomal RNA [7]. We now present experiments that validate the assumption of functionally equivalent SAM pools for cytoplasmic and mitochondrial RNA in a cell culture system.