Transcriptional and post-transcriptional processes regulate gene expression in oxygen-deprived roots of maize

To investigate the regulation of gene expression in maize (lea mays L.) in response to oxygen deprivation (flooding), we quantitated run-on transcription in isolated nuclei, steady-state mRNA accumulation and mRNA loading with ribosomes for seven genes that encode proteins synthesized predominantly in oxygen-deprived roots (anaerobic polypeptides) and seven genes that encode proteins synthesized in aerobic roots (aerobic polypeptides). Run-on transcription of anaerobic polypeptide genes was induced in response to oxygen deprivation and run-on transcription of the aerobic polypeptide genes continued during the stress treatment. The increased accumulation of mRNAs that encode anaerobic polypeptides occurred concomitant with the induction of gene transcription and efficient association of these mRNAs with large polysomes. The steady-state accumulation of aerobic polypeptide mRNAs was within twofold of aerobic levels and in a number of cases fewer ribosomes were loaded per transcript. These results demonstrate that selected synthesis of anaerobic polypeptides involves transcriptional as well as significant post-transcriptional regulation of gene expression. The repression of synthesis of many aerobic polypeptides occurs without interruption of gene transcription and is due to translational regulation and possibly the sequestration of mRNAs on mRNPs. Ribosome loading patterns indicated that this translational control occurs at both initiation and post-initiation phases in a message-specific manner.