RNASE ACTIVITIES ARE REDUCED CONCOMITANTLY WITH CONSERVATION OF TOTALCELLULAR RNA AND RIBOSOMES IN O-2-DEPRIVED SEEDLING ROOTS OF MAIZE

The effect of O-2 deprivation on the activities of RNases and levels o f total cellular RNA and ribosomes in seedling roots of maize (Zea may s L.) was investigated. Sodium dodecyl sulfate-polyacrylamide gels con taining RNA were used to distinguish RNase isoenzymes by apparent mole cular mass. Since O-2 deprivation causes a decrease in cytosolic pH fr om approximately pH 7.4 to 6.4 and an elevation in cytosolic Ca2+, RNa se levels were examined in the physiological range of cytosolic pH and in the presence of Ca2+, Mg2+, Zn2+, ethylenediaminetetracetate, or e thyleneglycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid. The ac tivity of a number of RNases present in aerobic roots was reduced in r esponse to O-2 deprivation. Several RNases with a pH optimum of 6.4 we re rapidly downregulated by O-2 deprivation. Spectrophotometric assay of extracts revealed that RNase activity was higher at pH 6.4 than at 7.2, and ethylenediaminetetracetate-insensitive RNase activity decreas ed in response to O-2 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. Re gulation of RNase activity may facilitate the conservation of nontrans lating ribosomes and poorly translated mRNAs during O-2 deprivation.