POSTTRANSCRIPTIONAL REGULATION OF GENE-EXPRESSION IN OXYGEN-DEPRIVED ROOTS OF MAIZE

Maize (Zea mays L.) seedling roots undergo transient changes in gene expression in response to O-2 deprivation (hypoxia or anoxia). Previous research has demonstrated the induction of transcription of alcohol dehydrogenase-1 (adh1) and implicated post-transcriptional and translational processes in the regulation of gene expression. In this study, the effects of O-2 deprivation on expression of mRNA encoding ADH1 and an aerobically synthesized protein, the mitochondrial adenine nucleotide translocator (ANT) were compared. The transcription of adh1 in isolated nuclei and steady-state level of poly(A)(+) mRNA encoding ADH1 increased in response to O-2 deprivation. In contrast, transcription of ant was not significantly affected, whereas the steady-state level of ANT mRNA steadily decreased under O-2 deprivation. To further understand the contrasting effect of low-O-2 on the stability of ADH1 and ANT mRNAs, post-transcriptional processes of gene expression were examined. RNA blot analysis of sucrose density gradient fractions of ribosomes revealed that ADH1 and ANT mRNAs are associated with polyribosomes under aerobic and O-2-deprived growth conditions. However, the number of ribosomes loaded per ANT mRNA was significantly reduced during O-2 deprivation. Immunoprecipitation analysis showed that synthesis of ANT was impaired relative to accumulation of ANT mRNA in O-2-deprived roots. The differences in synthesis of ADH1 and ANT in O-2-deprived roots appears to be due not only to increased transcription of adh1, but to competitive differences between transcripts for translation initiation factors and differences in ability to elongate during translation.