Differential expression and regulation of translation initiation factors ‐4E and ‐iso4E during maize germination

Messenger RNA translation is a tightly regulated event in maize axes during germination. To obtain a deeper understanding of this process, the present research was centered on the expression regulation of the translation initiation factor eIF‐4E within this period. Two maize eIF‐4E isoforms were isolated by mGTP‐Sepharose affinity chromatography. The eIF‐iso4E protein content remained at a constant level in the axes during the first 24 h of maize germination, whereas the eIF‐4E level increased after 12 h of germination as indicated by western blot analysis. Fast in vivo eIF‐iso4E, but not eIF‐4E, de novo synthesis was found within 6–12 h of germination. Northern blot analysis of total RNA from ungerminated axes with cDNAs encoding either ‐iso4E or ‐4E indicated that both transcripts were present in the stored mRNA pool of maize axes. Transcription for any of these mRNAs was not detected before 12 h of germination. Northern blots of polysomal RNAs indicated that the eIF‐iso4E stored transcript is selectively recruited into polysomes for translation as early as 6 h of germination, whereas this does not occur for the eIF‐4E transcript. The above data demonstrate differential 4E isoform expression regulation during maize germination. Expression of the eIF‐iso4E protein appears to be translationally controlled, whereas the expression of the eIF‐4E protein might be regulated at the transcriptional level.