Mode of translational activation of the catalase (cat1) mRNA of rye leaves (Secale cereale L.) and its control through blue light and reactive oxygen

The enzyme catalase (EC 1.11.1.6) is inactivated by light and must be continuously replaced by new synthesis in order to maintain a constant enzyme activity in leaves. In winter rye leaves (Secale cereale L.) posttranscriptional mechanisms determine the rate of new catalase synthesis, including a light-controlled reversible modification of the catalase cat1 mRNA by methylation which greatly enhanced its translation efficiency. The specificity and regulation of this mRNA activation were further investigated. The translation efficiency of the rye cat1 mRNA was much more enhanced by N-7 methylation of the cap than that of an lhcb transcript. Investigations with truncated rye cat1 mRNAs indicated that the translational enhancement resulting from N-7 cap methylation did not require the presence of specific sequences of cat1 5'- and 3'-untranslated regions. Translational activation of the cat1 mRNA in rye leaves was independent of photosynthesis and most effectively induced by blue light. Peroxides (H2O2, tertiary butyl hydroperoxide) and conditions enforcing an H2O2 accumulation in the leaves (aminotriazole, paraquat) also caused an activation of the cat1 mRNA. A search for further signalling systems controlling the replenishment of inactivated catalase in light suggested that an inositol-1,4,5-triphosphate-mediated liberation of Ca2+ from internal stores and a protein phosphatase played some role. However, these signalling systems did not affect the activation of the cat1 mRNA. After removal of Ca2+ by EGTA the cat1 mRNA was rapidly degraded.