Evaluation of Translational Control Mechanisms in Response to Oxygen Deprivation in Maize

Abstract

Numerous changes in gene expression occur in response to flooding (oxygen deprivation, i.e., anoxia and hypoxia) in seedling roots of maize (Zea mays L.) and other plants. Increased de novo transcription of anaerobic polypeptide (ANP) genes is responsible, in part for increased production of glycolytic and fermentative enzymes, such as alcohol dehydrogenase-1. There is increasing evidence that regulation of mRNA translation plays an important role in the expression of ANP genes during oxygen deprivation. By quantitative analysis of ribosomal complexes, we demonstrated a dramatic decrease in polysomes and an increase in monosomes in maize seedling roots deprived of oxygen, indicative of regulation of translational initiation. We report that oxygen deprivation causes dynamic changes in the phosphorylation status of the eukaryotic initiation factors (eIF) eIF4E, eIF4A, and eIF4B. By affinity purification of initiation complexes with 7mGTP and poly(A) resins, we demonstrate that a reduction in pH, which occurs in the cytosol in response to this stress, affects the assembly of mRNA 5"-cap and 3"-tail-binding complexes. We also describe oxygen deprivation-induced changes in phosphorylation of ribosomal protein S6, ribosomal 12-kD P-proteins and eukaryotic elongation factor-2 (eEF2). A model is presented that considers the implication of modifications in translational machinery in the interactions between the 5"-cap and 3"-tail of the mRNA that facilitate initiation and eEF2 GTPase activity which promotes elongation.