Effect of X-Irradiation on Nucleic Acids in Zea mays: I. Nucleic Acid Distribution and Metabolism in the Subcellular Components of Corn Seedlings Grown from Normal and X-Irradiated Seeds

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Abstract
X-Irradiation of dry Z. mays seeds decreases the content of microsmal RNA in the meristematic region of seedlings germinated from them. Meristematic cells from unirradiated seeds are characterized by a high level of microsomal RNA, whereas in maturing cells the majority of the RNA is associated with the mitochondria. Differential untra-centrifugation and RNA analysis show that exposure of seed to X-rays apparently prevents the synthesis of "new" microsomes as ordinarily occurs with cell division but does not disrupt the metabolism of microsomes during expansion of cells in the root tip. This results in an increase in mitochondrial RNA. The RNA of the different subcellular fractions appear to have dissimilar nucleotide compositions. Microsomal RNA contains a higher amount of GMP than the RNA of the other fractions, whereas the soluble RNA contains less GMP and more UMP. X-Irradiation of Z. mays seeds enhances the incorporation of precursors into the nucleic acids of all subcellular components of growing roots. Results show that the nuclei incorporated more precursors in vivo than the other cellular components. Generally, there is a greater incorporation of precursors into the RNA of the heavy mitochondria than into the light mitochondrial or soluble RNA fractions. The least amount of incorporation is into microsomal RNA. The incorporation of P32 in the RNA of the cytoplasmic components of intact roots is greater in the basal portion of the root (maturing cells) than in the meristematic zone. X-Irradiation inhibits the incorporation of amino acids into the protein of nuclei in vivo, but the incorporation is accelerated in vitro.