RNase Activities Are Reduced Concomitantly with Conservation of Total Cellular RNA and Ribosomes in O2-Deprived Seedling Roots of Maize

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Abstract
The effect of O2 deprivation on the activities of RNases and levels of total cellular RNA and ribosomes in seedling roots of maize (Zea mays L.) was investigated. Sodium dodecyl sulfate-polyacrylamide gels containing RNA were used to distinguish RNase isoenzymes by apparent molecular mass. Since O2 deprivation causes a decrease in cytosolic pH from approximately pH 7.4 to 6.4 and an elevation in cytosolic Ca2+, RNase levels were examined in the physiological range of cytosolic pH and in the presence of Ca2+, Mg2+, Zn2+, ethylenediaminetetracetate, or ethyleneglycol-bis([beta]-aminoethyl ether)-N,N[prime]-tetraacetic acid. The activity of a number of RNases present in aerobic roots was reduced in response to O2 deprivation. Several RNases with a pH optimum of 6.4 were rapidly down-regulated by O2 deprivation. Spectrophotometric assay of extracts revealed that RNase activity was higher at pH 6.4 than at 7.2, and ethylenediaminetetracetate-insensitive RNase activity decreased in response to O2 deprivation. The decrease in RNase activity was correlated with no loss of total cellular RNA or ribosomes, despite a 4-fold decrease in run-on transcription of rRNA in isolated nuclei. Regulation of RNase activity may facilitate the conservation of nontranslating ribosomes and poorly translated mRNAs during O2 deprivation.