Regulation of catalase synthesis during its light-induced turnover in leaves

Abstract

The enzyme catalase, which detoxifies the H2O2 produced during photorespiration and thus represents an important step of photoprotection, is generally light-sensitive. In leaves catalase has a light-induced turnover. However, usually a constant level of activity is maintained because the loss by inactivation is continuously replaced by de novo synthesis. Mechanisms by which the rate of catalase synthesis is attuned to fluctuating light conditions, were analysed in mature rye leaves (Secale cereale L.). Light-modulated changes of the rate of synthesis were not related to changes in the amounts of mRNA but determined by posttranscriptional controls. Conditions for the translation of catalase mRNA were investigated in vitro with poly[A]+RNA from rye leaves in a cell-free wheat germ lysate. Incorporation into catalase was visualized by fluorography after immunoprecipitation and electrophoretic separation. The rate of catalase synthesis was determined by the availability of the heme cofactor. Furthermore, the translational activity of the catalase mRNA was reversibly changed and attuned to the light conditions to which the leaves were exposed, prior to RNA extraction. In darkness the translational activity of the catalase mRNA declined (half-life: 2h). Light induced dose-dependent increases. The light-induced increases were not prevented when the accumulation of new catalase mRNA was blocked by cordycepin. The change of the translational activity must be due to some reversible modification of the existing mRNA. The translational activity of the catalase mRNA depended on the methylation pattern of the cap structure.